History of life on earth

Chapter 5                                    HISTORY OF LIFE ON EARTH

Summary of Chapter 5: EVOLUTION, D. Futuyma, 2nd Ed., 2009.

When studying this chapter keep in mind the following important patterns:

1. Climates and the distribution of oceans and continents have changed over time.
2. New forms of organisms have appeared and others have become extinct.
3. Extinction rates have been particularly high sometimes; these are called mass extinctions.
4. Especially after mass extinctions, the diversification of higher taxa has sometimes been relatively rapid.
5. The diversification of higher taxa has included increases both in the number species and in the variety of their form and ecological habits.
6. Extinct taxa have sometimes been replaced by unrelated ecologically similar taxa.
7. The ancestral members of related higher taxa are often morphologically more similar to one another, and more ecologically generalized, than their descendants.
8. Of the variety of forms in a higher taxon that were present in the remote past, usually only a few have persisted in the long term.
9. The geographic distributions of many taxa have changed greatly.
10. Over time, the composition of the biota resembles that of the present.

 

BEFORE LIFE BEGAN – some paragraphs of this section are repeated from chapter 4

The origin of the universe is shrouded in mystery.

• Science does not have the answer at present time to explain or describe how the first space and matter arose.
• Laws of nature describe the behavior of matter. Where do these laws come from?

 

Big Bang Theory (hypothesis?).

• The universe was a very concentrated mass of matter/energy on a very distant past.
• This mass exploded ("big bang") and hydrogen was formed.
• Hydrogen formed new elements, stars and galaxies.
• Galaxies are moving away from each other, expanding.
• The Big Bang assumes that energy, space and time already existed. The Big Bang makes no reference to where they came from. 
• The Big Bang may have occurred about 14 billion years ago.

 

Condensation or nebular theory

• Fragmentation of an interstellar cloud forming a nebula.
• Contraction and flattening of the nebula.        
• Condensation of the nebular material into protoplanets and meteorites.
• Solidification of planets and formation of our solar system about 4.6 billion years ago.
• The large condensing mass at the center became the sun.
• During condensation, elements scattered according to a density gradient with the heavy masses forming the planets near the sun and the lighter masses forming the outer planets.
• http://geol.queensu.ca/museum/index.php?option=com_content&view=article&id=50&Itemid=57

 

Earth Chronology, an approximation:

     4.6 BY (Billion Years ago) Solar system and the earth formed
4.5 BY the earth cooled and water condensed; formation of oceans
4.06 BY there were masses of dry land; formation of oldest KNOWN rocks known.
4 - 3.8 BY life originated; small protocontinents
3.5 BY oldest known fossils; first bacterium found in Australia.
2.5 BY photosynthesis, oxygen accumulated
1.5 BY first eukaryotes
700 MY soft-bodies multicellular life
540 MY hard-bodied multicellular life; beginning of the Phanerozoic eon; many modern phyla
appeared.

Interesting site: http://www.julesberman.info/chronos.htm
http://anthro.palomar.edu/earlyprimates/early_1.htm
http://pubs.usgs.gov/gip/geotime/contents.html

 

THE EMERGENCE OF LIFE

Many scientist consider life as a group of molecules that can capture energy from the environment, use that energy to replicate itself, and therefore, capable of evolving.

Living and semi-living things might have originated more than once, we can be quite sure that all organisms we know of stem from a single common ancestor because they all share certain features that are arbitrary as far as we can tell.

Some shared features of living organisms:

• All use L optical isomers of amino acids
• The genetic code
• Same method of replication
• Same method of protein synthesis
• Same basic metabolic reactions

SOLAR SYSTEM PREREQUISITES FOR THE ORIGIN OF LIFE.

NOTE: most of the information in this section is not found in your textbook.

Our planet possessed the essential prerequisites for the development of life.

• A moderate size sun that provided a steady rate of emitted radiation over a long period of time to allow the development of life.

 

• A variety of elements available to form organic molecules (H, O, N, C, S, P, Ca), and the ability of carbon to form four covalent bonds and different kinds of large, stable, three-dimensional molecules.

• The orbit of the earth is nearly circular thus maintaining a uniform distance from the sun and temperature extremes are eliminated.

 

• Large amounts of water present on the surface of the earth. Water is an excellent solvent that enables acids, bases and salts to ionize and react. It remains liquid over a wide range of temperatures. Its solid phase floats on the liquid phase.

• Large amount of hydrogen compounds that allowed protons to be donated to carbon.

 

• There was molecular preadaptation for the biochemical processes leading to the formation of biomolecules.
• Oparin and Haldane (1920s) independently proposed that the primitive atmosphere of the Earth was reducing and that organic compounds formed in such an atmosphere might be similar to those presently used by living organisms.

• The early atmosphere contained very little oxygen and was rich in hydrogen (H2) and compounds that can donate hydrogen atoms to other substances: a reducing atmosphere.

 

Oparin's and Haldane's ideas inspired the famous Miller-Urey experiment, which in 1953 began the era of experimental prebiotic chemistry.

There are serious doubts at present that the original atmosphere of the Earth was reducing. Rocks from the Archaean contain ferrous and ferric oxides, and sulfates, all of which contain oxygen.

THE MILLER EXPERIMENT

Harold C. Urey of the University of Chicago and Stanley L. Miller, a graduate student in Urey's laboratory, wondered about the kinds of reactions that occurred when the earth was still enveloped in a reducing atmosphere.

In 1953, Stanley Miller demonstrated that amino acids and other organic compounds could be synthesized spontaneously from hydrogen gas, ammonia, methane and other compounds presumed to have been present in the second atmosphere of the Earth if energy is provided.

Miller found that as much as 10 percent of the carbon in the system was converted to a relatively small number of identifiable organic compounds, and up to 2 percent of the carbon went to making amino acids of the kinds that serve as constituents of proteins.

Variants of Miller’s experiment are many. The results have been similar to Miller’s, a large number of organic molecules, some found in living organisms and others not.

In reproducing Miller’s experiment or its variants, it is important to keep O2 out of the reaction. Aldehydes and cyanides are first products, and then more complex organic compounds are synthesized.

Glycine was the most abundant amino acid, resulting from the combination of formaldehyde (CH2O), ammonia and hydrogen cyanide. A surprising number of the standard 20 amino acids were also made in lesser amounts.

A variety of organic compounds are produced when a heated mixture of carbon monoxide and hydrogen is passed over a catalyst. Adding ammonia produces purine and pyrimidine nucleotide bases that the Miller reactions do not produce.

Simple organic molecules have been found in carbonaceous meteorites called chondrites.

• The Murchison meteorite that fell in Australia in 1963 contained 80 kg of carbonaceous material and 1% of this material is organic carbon.
• The interior portion of this meteorite contains fatty acids up to eight carbons long.
• Practically all the amino acids found in the meteorite, protein and non-protein, are similar to the amino acids produced by sparking mixtures in laboratory experiments.

 

Since then, workers have subjected many different mixtures of simple gases to various energy sources. The results of these experiments can be summarized neatly:

• Under sufficiently reducing conditions, amino acids form easily.
• Conversely, under oxidizing conditions, they do not arise at all or do so only in small amounts.

 

Requirements for polymerization:

• Adequate concentration of monomers.
• Dehydration
• Protection against decomposition due to external factors.

 

Most polymerization involves the removal of water (dehydration) from the condensing molecules.

Condensing agents are molecules that help condensation (e.g. formation of a peptide bond) by removing water from the reactants (e.g. two amino acids) and releasing energy during the hydration of the condensing agent. The result is a dipeptide and a new organic molecule derived from the hydration of the condensing agent.

• Cyanide compounds have been identified as condensing agents.

 

            e. g. cyanamide (CN2H2), cyanogen (C2H2), cyanic acid (HCN), cyanoacetylene (C2N2H).

• Condensing agents provide the energy for the formation of dipeptides and other dimers.

 

e. g. formation of dipeptide bond can couple to the hydrolysis of cyanamide.

• Condensing agents accept water resulting from the condensing molecules: dehydration process.

 

Cyanogen and cyanamide can cause nucleotides to form by the phosphorylation of adenosine, uridine, and cytosine.

Oró had found that heating a mixture of hydrogen cyanide and ammonia in an aqueous solution could yield adenine.

Later studies established that the remaining nucleic acid bases could be obtained from reactions among hydrogen cyanide and two other compounds that would have formed in a reducing prebiotic atmosphere: cyanogen (C2N2) and cyanoacetylene (HC3N).

• The heating of aqueous solutions of ammonium cyanide produces adenine.

 

• Cyanoacetylene reacts with cyanates (e.g. urea) to produce the pyrimidine cytosine.

• Several researchers have proposed mechanisms for the formation of thymine and uracil from simpler cyanic molecules.

 

The phosphorylation of adenosine, uridine and cytosine can be accomplished in the presence of cyanogen and cyanamides. The product is adenosine monophosphate or AMP.

 

Thermal energy was probably the energy originally used by the early protocells.

Heat is unreliable and varied from place to place.

Chemical energy providers allowed a constant and regulated source of energy to be used in controlled and localized reactions.

Organic catalysts had to evolve to restrict reactions to the right time and place.

The cell developed a system that would provide energy in a controlled fashion when it is needed.

Proteins add speed to the process of catalysis.

THE RNA WORLD

"RNA came first. DNA and proteins came after."

In the late 1967 Carl R. Woese of the University of Illinois, Francis Crick, then at the Medical Research Council in England, and Leslie Orgel then working at the Salk Institute for Biological Studies in San Diego independently suggested a way out of this difficulty.

They proposed that RNA might well have come first and established what is now called the RNA world.

• A world in which RNA catalyzed all the reactions necessary for a precursor of life's last common ancestor to survive and replicate.

 

In 1983 Thomas R. Cech of the University of Colorado at Boulder and, independently, Sidney Altman of Yale University discovered the first known ribozymes, enzymes made of RNA.

Ribozymes are RNA fragments that posses catalytic properties.

Ribozymes can cut, splice and elongate other oligonucleotides.

"One can contemplate an RNA world, containing only RNA molecules that serve to catalyze the synthesis of themselves" (W. Gilbert, Nature, 319:618, 1986).

Recent experiments have shown that clay particles with RNA adsorbed onto their surface can catalyze the formation of lipid envelope, which in turn can catalyze the polymerization of amino acids into polypeptides.

Some macromolecules may have evolved that catalyzed the replication of each other.

Some scientists think that within the RNA world evolution existed because replication and natural selection of molecules can occur.

Long RNA sequences would not replicate effectively because the mutation rate would be too high for them to maintain any identity.

Replication was not exact.

Some oligonucleotides could replicate themselves.

The first “genes” need not have had any particular base pair sequence.

•  

Before proteins arose there were only genotypes without any particular base sequence.

Now a big jump...assuming the existence of some kind of template and polymerizing enzyme…

An RNA ribozyme binds to a cofactor made of an amino acid and short oligonucleotides, which have been joined by another ribozyme that joins specific oligonucleotides to amino acids according to a primitive code.

Amino acid and short nucleotide formed a cofactor, with the help of a ribozyme.

• Amino acid + cofactor + oligonucleotide help by a ribozyme in self replication

 

• The template could have a cofactor-ribozyme complex (Fig 5.2).

Many contemporary coenzymes have nucleotide components.

Selection could have led to the development of…

• The oligonucleotide component of the cofactor evolved into the transfer RNA
• The ribozyme evolved into the ribosome.
• The string of AA became a catalytic protein, en enzyme.

 

PRECAMBRIAN LIFE

PROKARYOTES

The Archaean, before 2.5 billion years ago, and the Proterozoic, 2.5 BYA to 542 MYA, are together referred as the Precambrian time.

Bacteria-like microfossils and stromatolites date back to 3.5 BYA

These organisms are thought to have been anaerobic.

The early atmosphere is thought to have little oxygen.

When photosynthesis evolved in bacteria and cyanobacteria, oxygen was introduced in large amounts in the atmosphere.

As oxygen increased in the atmosphere, organisms developed the aerobic respiration and mechanisms to protect the cells against oxidation.

Three domains are accepted today: one of eukaryotes, Eukarya, and two of prokaryotes, Archaea and Bacteria.

DNA provides conflicting evidence about the relationship among and within the domains, implying that there was extensive lateral gene transfer among lineages during the early history of life.

See Fig. 5.4, page 105.

The prokaryotes that descended from the last common ancestor (LCA) diversified greatly in their metabolic capacities: photosynthetic, chemoautotrophic, sulfate-reducing, methanogenic, etc.

EUKARYOTES

What makes a eukaryote a eukaryote?

The eukaryotes are distinguished from prokaryotes by being more complex and having membrane systems and compartments in which the life functions are separated from one another.

These compartments are called organelles and are bound by membranes:

• Nucleus with a nuclear envelope that is continuous with endomembrane system, endoplasmic reticulum and dictyosomes.
• Cytoskeleton containing actin and tubulin, and many other interacting proteins.
• Other organelles are chloroplasts, peroxisomes, mitochondria, vacuoles, etc.

 

Unifying characteristics of eukaryotes:

• Cytoskeleton consisting of tubulin-based microtubules and actin-based microfilaments, and ancestrally including motile cell extensions called ‘flagella’ or ‘cilia’ that contain an axoneme of 9 peripheral microtubular doublets and 2 central microtubules.
• An endomembrane system that consists of endoplasmic reticulum, Golgi bodies, vacuoles, lysosomes, peroxisomes, and the nuclear envelope.
• Primary genome of each cell consisting of multiple linear chromosomes contained within a membrane-bound nucleus.  Following replication of the genome the chromosomes are segregated by the process of mitosis.  Cells in many species can have more than one nucleus.
• Mitochondria - organelles with diverse functions, usually including aerobic respiration, iron sulfur cluster assembly, and synthesis and breakdown of small molecules such as lipids and amino acids. Mitochondria are bounded by two membranes, and usually contain a small genome. They are the descendents of an alpha-proteobacterial endosymbiont.
• Translation machinery in the form of 80S ribosomes, each consisting of four molecules of RNA complexed with many proteins, and partitioned in a small (40S) and a large (60S) subunit.

 

Tree of Life: Patrick Keeling, Brian S. Leander, and Alastair Simpson
See: http://tolweb.org/Eukaryotes

 

Evolution of Eukaryotic Organelles:

• Endosymbiotic hypothesis proposed by Margulis (1993) and others.

 

Eukaryotic cells evolved from prokaryotic cells that incorporated other prokaryotic cells using the mechanism of endocytosis.

Eukaryotes evolved several times.

These ingested prokaryotes became endosymbionts.

• Chloroplasts developed from photosynthetic cyanobacteria.

 

• Mitochondria were formed from aerobic heterotrophic bacteria.

• Flagella, cilia, centrioles and other structures are made of microtubules and appear to be homologous. They arose by the capture of spirochete-like prokaryotes.

 

Endosymbiosis has evolved many times.

• The plastids of dinoflagellates and apicomplexans may represent tertiary and quaternary endosymbiosis.

 

The earliest eukaryotic fossil is about 1.5 billion years old.

There is molecular evidence that the eukaryotic level may have arisen about 2.7 billion years ago.

Eukaryotes diversified very rapidly and gave rise during the Precambrian to the kingdoms and phyla existing today.

See fig. 5.5, page 106.

There are several hypotheses about the evolution of multicellularity, the appearance of metazoans.

• A metazoan is a multicellular animal with tissues.

 

Multicellularity is prerequisite for large size and development of organ systems.

Multicellularity probably arose because of the advantage provided by the division of labor.

The evolution of tissues and organs required the evolution of cell signaling mechanisms, cell adhesion, and complex gene regulation.

Basic signaling mechanisms already existed in unicellular organisms but in eukaryotes genes may have multiple binding sites for different transcription factors, and contribute to diverse developmental pathways.

 

PROTEROZOIC

The term "multi-celled" when applied to a plant or animal means that the organism is made up of several different cell types. The first multi-celled animals (metazoa) evolved over 640 million years ago.

Some of the first animal fossils are from…

• Ediacaran Fauna (630 - 590 m.y.a.) from Australia's Ediacara hills, and other parts of the world, from the Late Proterozoic (575-542 m.y.a.) and Early Cambrian. This fauna is also known as the Vendian fauna.
• Mostly flat organisms that crawled on the ocean floor.
• They appeared to have lacked mouthparts and appendages for locomotion.
• Most of these animals may have given rise to cnidarians and bilaterians.
• There is little evidence that they were subject to predation.
• http://geol.queensu.ca/museum/index.php?option=com_content&view=article&id=53&Itemid=60

 

PALEOZOIC LIFE: THE CAMBRIAN EXPLOSION

For a summary of the rest of the chapter refer to page 76 in your textbook. There you will find the major evolutionary events listed next to the era and period.

See this website: http://geology.uprm.edu/Morelock/paleoceanog.htm#list

There was a low diversity of organisms in the Early Cambrian following the Ediacaran fauna.

• First 10 million years of the Cambrian.

 

The Cambrian Explosion followed this low diversified fauna, where many new forms appeared, including most of present day existing phyla.

• Next 10 to 25 million years.

 

At the beginning of the Cambrian, about 542 (590?) m.y.a. there was an explosive radiation of multicellular animals. There is no agreement as to what caused this rapid diversification.

• Rapid evolutionary changes occurred in the late Proterozoic and ended in the Early Cambrian.
• A change from mostly simple organisms to complex organisms comparable to present day animals. This happened in 40 million years, 545 - 525 m.y.a.
• Most of the fundamentally different body plans (Baupläne, German for “construction plan) known in animals apparently evolved in the Cambrian.
• http://evolution.berkeley.edu/evolibrary/article/cambrian_02
• http://www.fossilmuseum.net/Fossil_Sites/Chengjiang.htm
• http://www.fossilmuseum.net/Fossil_Sites/Chengjiang/Chengjiang-Fossils.htm

 

It was a period of great evolutionary innovation. Almost all metazoan phyla appeared in the Cambrian. The phylum Bryozoa appeared in the Ordovician (490-435 m.y.a.)

• All marine invertebrate phyla appeared during this time, with the exception of the bryozoa. Mollusks, arthropods, annelids, chordates, echinoderms, and representatives of most other modern phyla make their first appearance in Cambrian deposits.

 

• Hard parts and skeletons appeared that initiated changes in the location and attachment of tissues. These hard parts provided protection, support and teeth (for predators). These were jawless, armored fish informally called ostracoderms, but more correctly placed in the taxon Pteraspidomorphi.

• Here was the first appearance of limbs and segmented bodies.

 

• The first organisms to be predators, and to develop shells, jaws, claws, and teeth appeared here as well.

• Jawless fish, eyes, gill pouches, segmented muscles, and notochord appeared.

 

• By the Late Cambrian teeth appeared in conodonts.

• We see varied multicellular and complex life that spent most of the time on the muddy sea floor.  All creatures were aquatic.

 

• The rapid increase in phyla suggests that many new habitats opened at this time.

• http://www.silurian.com/geology/life.htm

 

Metazoan groups appeared...

• Proterozoic eon, Vendian period (650-590 m.y.a.): echinoderms and cnidarians.
• Cambrian (590 - 505 m.y.a.): sponges, annelids, crustaceans, hemichordates, brachiopods, mollusks, fishes, etc.
• Most of the Cambrian animals belong to extinct Classes.

 

Evolutionary molecular clocks, based on nucleotide sequence data, indicate that divergences among major Cambrian lineages had already begun about 1000 m.y.a.

Molecular phylogenetic studies show that animals are most closely related to unicellular choanoflagellates, which have cell-adhesion proteins and cell-signaling proteins similar to those of animals.

The Burgess Shale

• 505 – 510 mya when present North America was located on the Equator
• Located in Yoho National Park, British Columbia, Canada
• Very well preserved soft-bodied organisms.
• Many bizarre shapes that do not fit in our present system of classification, therefore, unrelated to our present organisms.
• Some fit into our present classification system.
• http://paleobiology.si.edu/burgess/burgessSpecimens.html
• http://paleobiology.si.edu/burgess/cambrianWorld.html

 

Hypotheses about the causes of the Cambrian explosion

These three hypotheses are not mutually exclusive.

Ecological causes:

• Predators feed on the most abundantly prey species thus reducing their numbers and letting others use the resources.
• Cropping of a dominant species opens many niches, which become occupy by species previously excluded.
• Increase in prey species leads to an increase in predators.

 

Geologic causes:

• Oxygen may have reached critical threshold at this time.
• Oxygen supports a more active metabolism, which is required by fast moving animals.

 

Genetic causes:

• Possible major evolutionary changes in regulatory genes (Hox genes) that govern differentiation of body parts.

• Ohno claims that early Cambrian animals possessed genes capable of forming oxygen cross-links in collagen, in ligaments and tendons; hemoglobin; and homeobox proteins that help orient body plans along a directional axis.
• The diversification of animals is associated with the evolution of Hox regulatory genes, which lead to variation in morphology.

 

By the end of the Cambrian radiation, the animal phyla were locked into developmental patterns that constrained evolution enough that no additional phyla evolved after that period.

• All animals share a common ancestor. In other words, the animal kingdom is monophyletic. Choanoflagellates are the most closely related unicellular organisms to animals.

 

• Sponges (phylum Porifera) are basal animals.

• They diverged early from the rest of the animal kingdom.
• Sponges have choanoflagellate cells.
• Tissues evolved only after sponges diverged from other animals – parazoans: beside the animals.

• The Eumetazoa is a clade of animals with true tissues.

• The common ancestor of all animals except the sponges evolved true tissues.
• The basal eumetazoans are the phyla Cnidaria and Ctenophora, which are diploblastic and have radial symmetry (the radiata).
• All animals except Porifera and Placozoa are metazoans.

 

• The Bilateria is a monophyletic group that includes all animals with bilateral symmetry.

• Vertebrates and some other clades belong to the Deuterostomia.

• The name Deuterostomia refers to an animal developmental grade and also to a clade that includes the vertebrates.

 

Molecular systematists base their decision on the nucleotide sequences in the small subunit ribosomal RNA, and the sequence of the Hox genes. It is based on very few genes.

Morphological data produces a tree that divides the Bilateria clade into two groups: protostomes and deuterostomes.

Based on molecular data, some zoologists split the protostomes into two groups, the Lophotrochozoa and the Ecdysozoa.

See fig. 5.11, page 110.

PALEOZOIC LIFE: ORDOVICIAN TO DEVONIAN

Source of the following information:
http://www.ucmp.berkeley.edu/ordovician/ordolife.html
http://www.ucmp.berkeley.edu/silurian/silulife.html
http://www.silurian.com/geology/life.htm
http://www.ucmp.berkeley.edu/devonian/devlife.html
http://www.geol.umd.edu/~tholtz/G102/102lpal3.htm

MARINE LIFE

Many animal phyla diversified in the Ordovician (488 – 444 m.y.a) giving rise to many new classes and orders, including as many as 21 classes of echinoderms.

Trilobites, conodonts, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments.

The Ordovician is marked by the appearance of the oldest bony vertebrates whose appearance is completely known.  Typical Ordovician fish had large bony shields on the head, small, rod-shaped or plate-like scales covering the tail, and a slit-like mouth at the anterior end of the animal. Such fossils come from near-shore marine strata of Ordovician age in Australia, South America, and western North America.

The Ordovician ended with a mass extinction.

The jawless agnathans diversified in the following period, the Silurian (439 -408 m.y.a). The first gnathostomes, fish with jaws and paired fins, appeared during this period.

In the Devonian (408 – 354 m.y.a), the Osteichthyes, bony fishes, appeared and diversified. Two subclasses of Osteichthyes are recorded from this period for the first time:

• Sarcopterygii: the lobe-finned fishes that include lungfishes and rhipidistians.
• Actinopterygii: the ray-finned fishes that gave rise to the modern teleosts.

 

TERRESTRIAL LIFE

Perhaps the most eventful occurrence of the Ordovician was the colonization of the land.

Remains of early terrestrial arthropods are known from this time, as are microfossils of the cells, cuticle, sporangia and, spores of early land plants.

Silurian (444 to 416 mya): Vascular plants existed in the mid-Silurian. Most Silurian plant fossils have been assigned to the genus Cooksonia, a collection of branching-stemmed plants, which produced sporangia at their tips. None of these plants had leaves, and some appear to have lacked vascular tissue.

Also from the Silurian of Australia comes a controversial fossil of Baragwanathia, a lycophyte. If such a complex plant with leaves and a fully-developed vascular system was present by this time, then surely plants must have been around already by the Ordovician.

In any event, the Silurian was a time for important events in the history of evolution, including many "firsts," that would prove highly consequential for the future of life on earth.

By the start of the Devonian (416 – 359 mya), however, early terrestrial vegetation had begun to spread. These plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all.

• They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall.
• Alternation of generations was established by the end of the Devonian, with both generations showing equal complexity.

 

By the Late Devonian, lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many are rather tall plants.

• The progymnosperm produced some of the world's first forests.

 

By the end of the Devonian, the first seed plants had appeared.

• Evolution of the seed took place in the late Devonian.

 

The earliest terrestrial arthropods are known from the Silurian.

They fall into two major groups:

• The Arachnida include the terrestrial chelicerates that everyone is familiar with, and that nearly everyone would rather not be too familiar with: spiders (Aracneae), ticks and mites (Acari), and scorpions (Scorpiones).
• The earliest mandibulate centipedes and millipedes

 

The Sarcopterygii appeared in the Devonian, about 408 m.y.a. They included a group of fish called rhipidistians.

Rhipidistians had a tail fin as well paired fins that were fleshy. They also had gills and lungs.

Ichthyostegid amphibians, which were the first terrestrial vertebrates and the first tetrapods, evolved from lobe-finned rhipidistians fishes late in the Devonian.

Ichthyostega from the Devonian in Greenland had all the characteristics – tail fin, braincase, dermal skull bones, and lateral line canals, structure and distribution of teeth – as the rhipidistians.

They differ from rhipidistians in having larger pectoral and pelvic girdles and fully developed tetrapod limbs.

The proximal bone limbs are directly homologous to those of the rhipidistians but they had definite digits.

The transition from rhipidistians to ichthyostegids is marked by mosaic evolution – the limbs evolved faster than the skull and teeth – and by gradual change of individual features –the limbs are intermediate between those of lobe-finned fishes and later amphibians.

 

PALEOZOIC LIFE: CARBONIFEROUS AND PERMIAN

Sources of the information below:
http://www.ucmp.berkeley.edu/carboniferous/carblife.html
http://www.palaeos.com/Paleozoic/Permian/Permian.htm
http://www.geol.umd.edu/~tholtz/G102/102lpal4.htm
http://en.wikipedia.org/wiki/Synapsida

TERRESTRIAL LIFE

Landmasses formed in the southern hemisphere, the supercontinent called Gondwanaland during the Carboniferous (359 – 299 m.y.a).

Smaller land masses were found in the northern hemisphere.

Pangaea, the massive supercontinent that existed during the late Paleozoic and early Triassic, began forming during the Carboniferous.

Swamp forests as well as terrestrial habitats became common and widespread.

Widespread tropical climates favored the development of extensive swamp forests.

• In the swamp forests, the vegetation was marked by the numerous different groups that were present. Seedless plants such as lycopsids were extremely important in this community and are the primary source of carbon for the coal that is characteristic of the period.
• The lycopods underwent a major extinction event after a drying trend, most likely caused by the advance of glaciers, during the Westphalian-Stephanian boundary in the Pennsylvanian period.
• Ferns and sphenopsids became more important later during the Carboniferous, and the earliest relatives of the conifers appeared.

 

The first land snails appeared, and insects with wings that can't fold back such as dragonflies and mayflies flourished and radiated. These insects, as well as millipedes, scorpions, and spiders became important in the ecosystem.

The trend towards aridity and an increase in terrestrial habitat lead to the increasing importance of the amniotic egg for reproduction.

The earliest amniote fossil was the lizard-like Hylonomus, which was lightly built with deep, strong jaws and slender limbs. The basal tetrapods became more diverse during the Carboniferous.

The Permian extends from 299 to 250 mya.

The early Permian saw the continuation of the Carboniferous biomes, with polar tundra regions and warm wet tropical swamp forests. 

• But the drying climatic tendency during the mid Permian spelled death for the mighty swamp forests. 
• Water loving plants like Lycopods and Sphenopsids were greatly reduced in size, becoming mere shrubs. 

 

Plant life consisted mainly of ferns and seed-ferns, with new plants like conifers and ginkgos coming into prominence.

Seed plants began to diversify during the Permian.

• Cycads and conifers began to flourish.
• The large lycopods and horsetails became extinct.

 

In the Permian, the first group of insects with complete metamorphosis evolved, including beetles, primitive flies, and the ancestors of caddisflies, moths and butterflies.

It was the amniotes that took over as the dominant land animals. 

Although there were a number of different types of amniotes, the largest and most diverse belonged to the Synapsida, which were ancestral to the mammals.

The synapsids gave rise to the diapsids (Diapsida), the major reptilian stock whose descendants dominated the Mesozoic era.

The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian and went on to become the dominant large terrestrial animals during the latter half.

They were by far the most diverse and abundant animals of the Middle and Late Permian, including a diverse range of herbivores and carnivores,

Only a few therapsids (synapsids) survived the great Permian extinction.

 

AQUATIC LIFE

Shallow, warm, marine waters often flooded the continents during the Carboniferous.

Attached filter feeders such as bryozoans, particularly fenestellids, were abundant in this environment, and the sea floor was dominated by brachiopods.

Trilobites were increasingly scarce while foraminifers were abundant.

The heavily armored fish from the Devonian became extinct, being replaced with fish fauna that look more modern.

The deltaic environment supports fewer corals, crinoids, blastoids, cryozoans, and bryozoans, which were abundant earlier in the Carboniferous.

Freshwater clams first appear along with an increase in gastropod, bony fish, and shark diversity.

The Permian warm shallow oceans swarmed with many kinds of life, basically very similar to Carboniferous forms. 

Sedentary organisms like stromatolites, algae, foraminifers, sponges, corals, bryozoa, and brachiopods (including the spiny Edriosteges), built great reefs which in turn provided homes and shelter for active animals like ammonoids, nautiloids, gastropods and fish.

The Permo-Triassic Extinction:

The Permian ended 251 m.y.a with the greatest mass extinction ever recorded

Largest mass extinction of the Phanerozoic. Total of all Permo-Triassic event may be 96% of species and 52% of the families of marine invertebrates became extinct.

Many land animals and plant became extinct and the aspect of the landscape changed considerably.

After the event, the Paleozoic marine evolutionary fauna becomes subordinate to the Modern marine evolutionary fauna.

Paleontologists do not agree on the cause of this massive extinction event. One candidate is the volcanic eruptions that created the Siberian traps, which released massive amounts of poisonous gasses, which in turn caused large amounts of acid rains.

 

MESOZOIC LIFE

Source of the information below:
http://www.rocksandminerals.com/geotime/geotime.htm
http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch12/chapter12-07.html
http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch12/chapter12-10.html
http://www.ucmp.berkeley.edu/mesozoic/cretaceous/cretlife.html

 

The Mesozoic era consists of the Triassic (251-200 m.y.a.), Jurassic (200-145 m.y.a.) and Cretaceous (145-65 m.y.a.) periods.

The Mesozoic is often called the Age of Reptiles and Age of Gymnosperms.

Pangaea began to break up at the beginning of the Jurassic with the formation of the Tethys Sea separating Asia and Africa.

This was followed by the separation of the northern land mass, Laurasia, from the southern land mass, Gondwanaland.

• Laurasia: North America, Greenland, Eurasia.
• Gondwanaland: South America, Africa, Antarctica, India, Madagascar, Australia and New Zealand.

 

Laurasia began to separate during the Jurassic (200-145 mya).

Gondwanaland began to separate during the Cretaceous (145-65 mya).

During the Mesozoic the sea level rose and there were many shallow continental seas.

The climate was warm and the polar regions were cool.

MARINE LIFE

The survivors of the Permo-Triassic extinction diversified.

Phytoplankton and zooplankton were present in Mesozoic seas.

Diatoms appeared in the Cretaceous.

Planktonic foraminiferans, ammonoids, and bony fishes continued to diversify during the Triassic.

• Paleozoic foraminiferans were bottom dwellers.
• Scleractinian corals (hard skeleton corals) replaced the tabulate and rugose corals, which disappeared at the end of the Paleozoic.

 

There was a mass extinction at the end of the Triassic that affected the ammonoids and bivalves.

These groups underwent another adaptive radiation during the Jurassic that began the “the Mesozoic marine revolution” in which crabs and bony fish with the ability to crush the shells of mollusks appeared.

Gastropods and bivalves rose to dominance during the Jurassic and Cretaceous.

Marine reptiles appear: ichthyosaurs and placodonts in the Triassic; plesiosaurs, mososaurs and sea turtles in the Cretaceous.

Two major types of fishes existed during the Mesozoic, the Chondrichthyes and the Osteichthyes.

Echinoderms became much more diverse during the Mesozoic than they had been during the Paleozoic.

The end of the Cretaceous is marked by the best known mass extinction, the K/T extinction.

Ammonoids, rudist mollusks, marine reptiles and many families of invertebrates and plankton became extinct.

TERRESTRIAL LIFE

Plants and Arthropods

The early Mesozoic was dominated by ferns, cycads, ginkgophytes, bennettialeans, and other unusual plants.

Modern gymnosperms, such as conifers, first appeared in their current recognizable forms in the early Triassic.

By the middle of the Cretaceous, the earliest angiosperms had appeared and began to diversify, largely taking over from the other plant groups.

By the late Cretaceous, most families of living insects, including ants and social bees, had evolved.

Insects and angiosperms developed an intimate relation involving pollination and may have helped each other’s diversification.

Complex biological communities formed by angiosperms, dinosaurs, insects and others existed by the end of the Cretaceous

Vertebrates

The major groups of amniotes are distinguished by the number of openings in the temporal region of the skull.

Modern amphibians appeared:

1. Frogs and toads (Anuria): The oldest known frog is from the Early Triassic of Madagascar.
2. Salamanders and newts (Urodela); The oldest known are from the Late Jurassic of Kazakhstan.
3. Limbless amphibians (Caecilians); although they resemble earthworms, they have as many as 200 vertebrae. The oldest is from the Early Jurassic of Arizona.

The diapsid reptiles with two temporal openings became the most diverse group of reptiles.

The diapsids evolved into two groups: lepidosauromorphs and archosauromorphs.

The lepidosauromorph reptiles evolved into modern suborders in the late Jurassic and into modern families in the Cretaceous.

• Snakes and lizards evolved in the Jurassic
• Tuatara evolved in the Triassic
• Marine reptiles called Eurapsyds

 

One group of lizards evolved into the snakes possibly in the Jurassic but their oldest fossil is from the Cretaceous.

The most interesting of Mesozoic reptiles were the archosaurs, a group of large diapsids, which includes crocodiles, extinct flying reptiles (pterosaurs), dinosaurs, thecodonts, and modern birds.

Archosaurs gave rise to the pterosaurs, the flying reptiles.

• The largest pterosaur was Quetzalcoatlus which had a 15.5 m (50 foot) wingspan. They have been found in the Upper Cretaceous of western Texas, and are the largest known flying vertebrate which ever lived on the Earth.

 

The thecodonts were the ancestors of the dinosaurs.

Thecodonts were small, agile reptiles with long tails and short fore-limbs; many were bipedal.

Dinosaurs evolved from the thecodonts (Archosaurs); they appeared in the Late Triassic.

Not all ancient reptiles are dinosaurs; only those belonging to the Orders Saurischia and Ornithischia are dinosaurs.

• Both orders arose in the Triassic and became diverse in the Jurassic.
• More than 39 families are recognized.
• They are distinguished by details of their hip.

 

The Saurischia included bipedal carnivores (theropods) and quadrupedal herbivores (sauropods).

• Birds and crocodilians are considered modern dinosaurs descendants of the saurischian branch.
• Archaeopteryx was in Late Jurassic strata of southern Germany.
• It closely resembles a theropod dinosaur with feathers.
• Feathered dinosaurs have been found recently in China: Sinosauropteyx, Caudipteryx, and Microraptor.
• Feathers are no longer considered a synapomorphy of birds, but only the opposable hind toe.
• Were feathers used first in insulation or in flight?

 

The Ornithischia included herbivores with numerous teeth.

• Ceratopsians, horned dinosaurs, lived in the Cretaceous.
• Duck-billed dinosaurs had mouth plates to chew on the vegetation.

 

The synapsids, with a single temporal opening, gave rise to the mammal-like reptiles (therapsids), which survived the Permian extinction.

Therapsids were small too moderate in size, with several mammalian skeletal traits.

• Differentiated teeth
• Legs beneath body

 

One group of therapsids became particularly common during the Triassic. These were the cynodonts (meaning "dog toothed").

Cynodont therapsids, which lived form the late Permian to the late Triassic, represent several steps in the approach toward mammals.

• In Morganucodon of the late Triassic and very early Jurassic, the teeth are typical of mammals.
• Morganucodon has both a weak articular/qudrate hinge and a fully developed mammalian articulation between the dentary and the squamosal. The articular and quadrate bones are sunk into the ear region, and together with the stapes, they closely approach the condition in modern mammals, in which these bones transmit sound to the inner ear.

 

Therapsids continued into the Triassic but became extinct in the Early Jurassic, after giving rise to the mammals.

Early mammals were rodent-like, and remained small throughout Mesozoic (smaller than housecats).

Most of the early mammals have left no direct descendants, but therian mammals, the stem group from which most living mammals are descended, are known from the early Cretaceous.

The ancient egg-laying mammals of the Jurassic are survived today by the platypus and equidnas of Australia and New Guinea.

The two major subclasses of living mammals, marsupials and eutherians (placental Mammals), appear in the fossil record during the early Cretaceous.

The Mesozoic ended 65 m.y.a with the K-T extinction.

• K/T extinction: K stands for Kreide, chalk in German.
• Cretaceous is derived from Creta, the Latin word for chalk.

 

But many groups of organisms, such as flowering plants, gastropods and pelecypods (snails and clams), amphibians, lizards and snakes, birds, crocodilians, and mammals "sailed through" the Cretaceous-Tertiary boundary, with few or no apparent extinctions at all.

See Fig. 5.21 on page 119.

CENOZOIC ERA

The Cenozoic consists of six epochs, from the Paleocene to the Pleistocene and Recent.

Traditionally, the first five epochs are referred to as the Tertiary period, 65 – 1.8 m.y.a, and the Pleistocene and Recent as the Quaternary period.

The last 10,000 years is sometimes referred to as the Holocene or Recent.

Some paleontologists divide the era into the Paleogene and Neogene periods, a different division of the era into periods.

“Gondwana began to break up in the mid- to late Jurassic (about 167 million years ago) when East Gondwana, comprising Antarctica-Madagascar-India-Australia, began to separate from Africa during the Middle Jurassic. South America began to drift slowly westward from Africa as the South Atlantic Ocean opened, beginning about 130 million years ago (Early Cretaceous) and resulting in open marine conditions by 110 million years ago. East Gondwana itself began to be dismembered as India began to move northward, in the Early Cretaceous (about 120 million years ago)… Australia began to separate from Antarctica perhaps 80 million years ago (Late Cretaceous), but sea-floor spreading between them became most active about 40 million years ago during the Eocene epoch of the Tertiary Period…New Zealand probably separated from Antarctica between 130 and 85 million years ago.”   http://en.wikipedia.org/wiki/Gondwana

• “The continent of Australia-New Guinea began to gradually separate and move north (55 million years ago)

 

• South America separating from West Antarctica some time during the Oligocene, perhaps 30 million years ago. Immediately before this, South America and East Antarctica were not connected directly, but the many microplates of the Antarctic Peninsula remained near southern South America acted as "stepping stones" allowing continued biological interchange and stopped oceanic current circulation.

• “South America was connected to North America via the Isthmus of Panama, cutting off a circulation of warm water and thereby creating the Arctic.” This occurred during the Pliocene, about 3.5 m.y.a.

 

• “The Red Sea and East African Rift are modern examples of the continuing dismemberment of Gondwana.”

                http://en.wikipedia.org/wiki/Gondwana

About 8 to 14 m.y.a, during the Miocene (23-5 mya), Africa made contact with southwestern Asia, India collided with Asia forming the Himalayas, and Australia moved northward, approaching southeastern Asia.

The Bering Land Bridge, Beringia, existed on and off during most of the Cenozoic. The last land connection between Siberia and Alaska dates to about 11,000 years ago, in the Pleistocene.

During the late Eocene (55 – 34 m.y.a) and Oligocene (34 – 23 m.y.a) there was a cooling and drying of the Earth creating extensive savannas. Glaciers appeared in Antarctica for the first time.

During the late Pliocene (11 – 3 m.y.a) temperatures began to drop and a series of glaciations began that persisted until the Pleistocene (3 m.y.a – 500 kyat).

The most recent Ice Age ended about 8,000 years ago.

 

AQUATIC LIFE

Cenozoic phytoplankton includes diatoms, dinoflagellates and coccolithophores, and rapidly diversified due to decrease competition caused by the great K-T extinction.

Foraminiferans and radiolarians were common zooplankton in the early Cenozoic.

The taxonomic composition of Cenozoic marine communities was quite similar to that of modern ones.

Teleosts continue to diversify becoming the most diverse group of aquatic vertebrates.

Sharks become the top predators of the seas after the extinction of the marine reptiles.

Sand dollars evolved from ancient forms; corals diversify.

Ancient whales or archaeocetes appeared in the Eocene (55-39 m.y.a) and evolved into the modern forms in the Oligocene (39-23 m.y.a).

• Archaeocetes had characteristics of land mammals that are absent in modern whales: nostrils at the tip of the nose; hind legs and pelvis attached to the sacrum; differentiated teeth, etc.
• Early archaeocetes were amphibious.

 

Several modern families of toothed and baleen whales evolved in the Miocene (23-5 m.y.a).

The first sirenians manatees and sea cows also appeared in the Eocene, 55 -39 m.y.a.

During the Pleistocene glaciations, the sea level dropped by as much as 120 m, and many mollusks became extinct.

TERRESTRIAL LIFE

Angiosperms continued to diversify and dominate through the Tertiary. Most modern families appeared in the Eocene (55 – 34 mya) or even earlier.

Plant distribution was strongly controlled by climate.

• As climate cooled through the Tertiary, plant distribution also shifted.
• This cooling began at the end of the Eocene, when temperatures dropped and aridity increased.

 

Many modern genera of insects existed in the Eocene (55-34 mya).

Savannas dominated by grasses become extensive in the Oligocene, 39-23 m.y.a.

The grasses, family Poaceae, underwent a major adaptive radiation at this time.

Many herbaceous plants evolve from woody ancestors.

Fully modern birds appear in the Early Tertiary and adapt to a large number of habitats.

• Marine (penguins)
• Flightless predators e.g. Diatryma
• Large flightless forms (Rheas, Ostrich, Emu)
• Air adapted (owls, hawks, ducks, etc.)

 

The largest family of birds, the Passerines or perching birds diversified greatly in the Miocene, 23-5 m.y.a.

Amphibians resemble modern forms, and are different from the large Paleozoic amphibians.

Surviving reptiles included the following: Turtles, Crocodilians, Lizards, Snakes and the tuatara, found on islands near New Zealand.

• The turtle lineage dates back to the Late Permian.

 

Snakes evolved from lizards in the Mesozoic but diversified in the Miocene and poisonous snakes make their appearance in this period.

ADAPTIVE RADIATION OF MAMMALS

DNA sequencing indicates that most mammalian Orders evolved and appeared in the late Cretaceous (145-65 m.y.a).

The major lineages within each Order diverged between 77 and 50 m.y.a.  See fig. 5.26, page 124.

Primitive mammals might have been nocturnal and insectivorous

During the middle to late Cretaceous period (80-65 m.y.a.), the three modern living groups of
mammals (placentals, marsupials, and monotremes) evolved.

• Eutheria, Metatheria and Prototheria

 

The southern continents (South America, Australia, and Antarctica) were separated from North America and Eurasia during most of the Cenozoic. As a result, distinctive assemblages of mammals developed on the southern continents, showing convergent evolution with northern hemisphere species.

The development of the Panama land bridge about 3.5 million years ago (during the Late Pliocene) led to the migration of mammals between North and South America.

• Marsupials (Metatherians) are thought to have evolved, along with placental (Eutherian) mammals, from Therian mammals.
• Marsupials diverged from Eutherian mammals approximately 90 million years ago.*
• Marsupials probably evolved in North America, expanded into South America and the Pacific rim of Asia. During this period of migration the North American marsupials became extinct, followed by extinctions in Europe during the Miocene epoch of the Tertiary period.
• Marsupials began to migrate to Australia and New Zealand from North America in the late Cretaceous or early Tertiary period. The route of migration crossed Antarctica and into Australia. As Australia broke off from Antarctica and moved northwards, its isolation from other landmasses was complete and the independent evolution of marsupials in Australia and New Zealand began.

            Source: http://www.nhc.ed.ac.uk/index.php?page=24.134.165.168.256
University of Edinburgh, Bernard E. Matthews and Patricia M. Preston, Joint Museum Curators, Edinburgh, October
2001.

*Note: this date varies with different authors from 135 to 120 to 90 m.y.a.

These monotremes split from the early mammals around 130 million years ago, probably in Australia, while it was still joined to Antarctica, as part of Gondwanaland. 

 Monotremes had dispersed through Antarctica into South America by the early Paleocene (65 million years ago), but are only found today in Australia, where they were safe from competition with the more advanced placental mammals.

Monotremes resemble reptiles and differ from all other mammals in that they lay shell-covered eggs that are incubated and hatched outside of the body of the mother.   

• Like reptiles, the ducts of the excretory system and the genital ducts open into a single external opening known as the cloaca (thus, the ordinal name Monotremata). 
• Like mammals, they have fur, a four-chambered heart, are warm-blooded, and nurse their young from specialized glands. 
• However, as might be expected in these primitive mammals, the four-chambered heart is, to an extent, incomplete, the body temperature averages lower than that of other mammals, and there are no nipples; rather, the milk is excreted through skin glands, with the young suckling the body fur to receive the milk.  The monotremes are represented today by two species of echidnas and one species of platypus.
• Source: http://www.bobpickett.org/evolution_of_mammals.htm

 

Placentals evolved in Laurasia some 120-90 m.y.a.

By the mid-Cretaceous (97-95 m.y.a.) marsupials appeared in North America and placentals appeared in North America and Asia. The oldest monotreme fossil is from the mid-Cretaceous (~100 m.y.a) of Australia.

By the late Cretaceous and early Tertiary, metatherians and primitive eutherians probably had replaced prototherian groups in South America, and metatherians had reached Antarctica.

South America began to drift away from North America and Eurasia during the mid Cretaceous.

Within 10 million years of the dinosaur extinction, nearly all modern placental groups have
emerged on the scene.

PRIMATE EVOLUTION

Paleocene 65.5–55.8 mya.

• First proto-primates evolved in the early Paleocene.
• Similar to squirrels and tree shrews in size and appearance.
• From Europe, North Africa and western North America.
• Arboreal with good eyesight.
• Plesiadapiformes

Eocene 55.8-33.9 mya

• Emergence of the early form of most placental mammals.
• Prosimian appearance and adaptive radiation
• Adapidae and Omonyidae
• In North America, Europe, Africa, and Asia. At this time they reached Madagascar.
• Brain and eyes become larger; snouts become smaller; foramen magnum begins to move from the back to the center of the skull.

Oligocene 32-23 m.y.a.

• The anthropoid level was already present 30 m.y.a. Probably originated about 40 m.y.a. in the late Eocene in Africa.
• The first monkeys appeared here.
• Apidium and Aegyptopithecus appeared.

Miocene 25-5 m.y.a.

• During the long Miocene (25 to 5 million years ago) the catarrhines underwent an explosion in diversity.
• Proconsul appears in Africa 20-14 mya.
• Dryopithecus appears 14 mya appears in Africa.
• Apes appear in this period by 20 m.y.a.

http://anthro.palomar.edu/earlyprimates/early_2.htm

OTHER MAMMALS

Rodents are related to Primates.

• The rodents became the most diverse order of mammals within the last 10 million years.

During the Miocene (24-5-m.y.a.) and Pliocene (5 - 2 m.y.a.) the world gets colder, polar ice begins to become prominent, tropical areas diminish and move to the more central latitudes, and grasslands and prairies flourish. In order to survive with the changing climate and corresponding habitats, many herbivores, and particularly the ungulates had to evolve new strategies. This interval is called the Age of Ungulates.

Horses, cattle, elephants, pigs, deer etc. had evolved by the end of the Pliocene (2 m.y.a.).

During the Eocene, the Perissodactyla diversified, and Artiodactyla appeared.

• Perissodactyla: odd toed ungulates, dwindled at the end of Eocene.
• Rhinoceroses, horses and tapirs
• The Artiodactyla diversified in the Miocene and diversified.
• Ruminants, camels, giraffes.

 

There is evidence that a group of artiodactyls related to the hippopotamuses became aquatic and eventually evolved into the cetaceans, dolphins and whales.

http://anthro.palomar.edu/earlyprimates/time_scale_of_Earth_1.htm

PLEISTOCENE EVENTS

The Pleistocene began about 1.8 million year ago and extend to the present.

By the beginning of the Pleistocene the continents had reached their present position.

• The Bering land bridge connected Siberia and Alaska.
• The isthmus of Panama connected North and south America.
• The Sunda shelf joined SE Asia with Indonesia and the Philippines.
• Australia and New Guinea were connected by a land bridge.

 

Pleistocene species were similar to those that live today.

Global temperatures began to droop during the Pliocene about 3 million years ago and fluctuations of global temperature occurred in 100,000-year cycles.

• The glaciers made four major advances and many smaller ones.

 

The most recent glaciation is called the Wisconsin glaciation, which reached its maximum 18,000 years ago and melted back between 15,000 and 8,000 years ago.

Sea levels drop by 100-120 meter exposing much of the continental shelf and connecting many land masses.

The global climate during these glacial episodes was generally drier and mesic and wet forests were restricted to small areas. Grasslands expanded.

These climatic changes affected the distribution of organisms.

Land bridges allowed the migration of species into new habitats, e.g. human migration from Asia to America.

Many species that were uniformly distributed became split into isolated populations.

Some of these isolated populations became new species.

Extinction of species occurred:

• Many tropical marine species died out.
• On land, many large-bodied mammals and birds became extinct: megafaunal extinction.
• There is the hypothesis that human contributed to the demise of large animals through the use of weapons.

 

Additional information and interesting sites:
http://www.archaeologyinfo.com/species.htm
http://www.wsu.edu/gened/learn-modules/top_longfor/timeline/timeline.html
http://en.wikipedia.org/wiki/Timeline_of_evolution
http://en.wikipedia.org/wiki/List_of_prehistoric_mammals
http://en.wikipedia.org/wiki/History_of_Earth
http://en.wikipedia.org/wiki/Timeline_of_human_evolution
http://www.efn.org/~jack_v/timeline.html
http://www.cr.nps.gov/history/online_books/geology/publications/bul/1291/sec3.htm

 

SUMMARY

PROTEROZOIC LIFE

Ediacaran Fauna

• 630 - 590 m.y.a. from Australia.
• Other parts of the world from Late Proterozoic 575-542 m.y.a. and Early Cambrian.

• Multicellularity
• Mostly flat organisms that crawled on the ocean floor.
• They appeared to have lacked mouthparts and appendages for locomotion.

 

PALAEOZOIC LIFE

CAMBRIAN – 542 to 490 m.y.a.

• Cambrian explosion
• Explosive radiation of multicellular animals.
• Most of present day existing phyla.
• All marine invertebrate phyla appeared during this time
• Hard parts and skeletons appeared
• First appearance of limbs and segmented bodies
• Jawless fish, eyes, gill pouches, segmented muscles, and notochord appeared
• Parazoans Eumetazoans

ORDOVICIAN - 490-444 m.y.a

• Trilobites, conodonts, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods.
• First appearance of Ostracoderms: armored and jawless.
• Colonization of the land: microfossils of the cells, cuticle, sporangia and, spores of early land plants.
• Ordovician ended with a mass extinction

SILURIAN – 444 to 416 m.y.a.

• The first gnathostomes, fish with jaws and paired fins, Placoderms.
• Diversification of agnathans and ostracoderms.
• Vascular plants: Genus Cooksonia, a collection of branching-stemmed plants, which produced sporangia at their tips.
• Lycophytes, a complex plant with leaves and a fully developed vascular system.

DEVONIAN – 416 to 359 m.y.a.

• Osteichthyes, bony fishes, appeared and diversified: Sarcopterygii and Actinopterygii
• Early terrestrial vegetation had begun to spread
• Alternation of generations was established by the end of the Devonian
• By the Late Devonian, lycophytes, sphenophytes, ferns, and progymnosperms had evolved.
• First forests.

• Early terrestrial arthropods: spiders and scorpions.
• Rhipidistians > Ichthyostegid amphibians > Amphibians

CARBONIFEROUS – 359 to 299 m.y.a.

• Gondwanaland is formed in the southern hemisphere.
• Smaller land masses found in the northern hemisphere
• Swamp forests common and widespread
• Lycopods were extremely important in this community and are the primary source of carbon for the coal that is characteristic of the period.
• Lycopods are replaced by ferns and horsetails (sphenopsids).
• First land snails appeared.
• Insects with wings that can't fold back such as dragonflies and mayflies flourished and radiated.
• Millipedes, scorpions and spiders became important in the ecosystem.

• Earliest relatives of the conifers appeared.

PERMIAN - 299 to 251 mya

• Pangaea is formed by the end of the Permian.
• Heavily armored fish from the Devonian became extinct.
• Freshwater clams first appear.
• Stromatolites, algae, foraminifers, sponges, corals, bryozoa, and brachiopods, built great reefs.  
• Increase in gastropod, bony fish, and shark diversity.
• Plant life consisted mainly of ferns and seed-ferns
• Seed plants began to diversify
• Large lycopods and horsetails became extinct
• Insects with complete metamorphosis evolved, including beetles, primitive flies, and the ancestors of caddisflies, moths and butterflies
• The amniotes that took over as the dominant land animals.
• Synapsid reptiles appear; ancestors of the therapsids, which gave rise to mammals.
• Therapsids become dominant and diverse, herbivores and carnivores.
• The Permian ended 251 m.y.a with the greatest mass extinction ever recorded

MESOZOIC LIFE

The Mesozoic is often called the Age of Reptiles and Age of Gymnosperms.

TRIASSIC - 251-200 m.y.a.

• Phytoplankton and zooplankton
• Planktonic foraminiferans, corals, ammonoids, and bony fishes continued to diversify.
• Two major types of fishes existed during the Mesozoic, the Chondrichthyes and the Osteichthyes.
• Marine reptiles appear: ichthyosaurs and placodonts.
• Mass extinction at the end of the Triassic that affected the ammonoids and bivalves.
• Modern gymnosperms, such as conifers appeared and become dominant.
• Reptiles diversify including first dinosaurs.
• First mammals.

JURASSIC - 200-145 m.y.a.

• Separation of Pangaea into the northern landmass, Laurasia, from the southern landmass, Gondwanaland.
• Laurasia began to separate during the Jurassic.
• Cycads, ginkgoes and tree ferns are common.
• Modern families of conifers flourish, e.g. pines, cypresses, yews.
• First birds, Archaeopteryx.
• Archaic mammals.
• Therapsids became extinct in the Early Jurassic, after giving rise to the mammals.

CRETACEOUS - 45-65 m.y.a.

• Gondwanaland began to separate during the Cretaceous.
• Diatoms appeared.
• Plesiosaurs, mososaurs and sea turtles appeared.
• The earliest angiosperms appeared and began to diversify.
• Insects and angiosperms developed an intimate relation involving pollination.
• Increase diversification of angiosperms, mammals, dinosaurs, and birds.
• Monotremes, marsupials and placental mammals evolve.
• Mass extinction at the end of the Cretaceous, including dinosaurs.

 

But many groups of organisms, such as flowering plants, gastropods and pelecypods (snails and clams), amphibians, lizards and snakes, birds, crocodilians, and mammals "sailed through" the Cretaceous-Tertiary boundary, with few or no apparent extinctions at all.

CENOZOIC LIFE

TERTIARY – 65 to 1.8 m.y.a.

• Phytoplankton including diatoms, dinoflagellates and coccolithophores, and rapidly diversified.
• Foraminiferans and radiolarians were common zooplankton. 
• Sharks become the top predators of the seas.
• Teleosts, modern fish, continue to diversify.
• Sand dollars evolved
• Corals diversify.
• Radiation of mammals, birds, snakes, pollinating insects, and snakes.
• Snakes evolved from lizards.
• Ancient whales or archaeocetes appeared in the Eocene (55-39 m.y.a).
• Fully modern birds appear in the Early Tertiary  
• Angiosperms continue to diversify.
• Grasses diversify as savannas are formed.
• Herbaceous plants evolve and diversify from woody ancestors.

 

QUATERNARY – 1.8 m.y.a. to present

• Extinction of large mammals.
• Appearance of hominids.
• Homo habilis, H. erectus, etc.
• Fire, art, agriculture and beginning of civilization.
• Modern flora and fauna dominated by mammals and angiosperms.

 

http://www.mnhnc.ul.pt/pls/portal/docs/1/335007.pdf