Evolutionary Timeline

The Evolutionary History of the Earth

Last updated: Dec 08, 2008

This site is intended to give a one-page summary of the history of the Earth from its origin 4.6 billion years ago to its demise from an expanding sun in about five billion years. The focus is on the interrelationship between evolution, continental configuration due to plate tectonics, and climate.

Geological Periods
Detailed Evolutionary Time line
Changes in the Atmosphere over Time
References

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Geological Periods The chart below summarizes the Earth's geological periods. Eras are linked into the detailed evolutionary time line below. The red line between periods in the Paleozoic and Mesozoic eras indicates a major mass extinction. The following icons are used: Map of how the Earth's continents were configured at that time, from PALEOMAP Project. Map from Paleogeographic Atlas Project, from the University of Chicago. (These PDF images may be slow to load) Paleomaps presented by Ron Blakey, Northern Arizona University. Link to a paleoclimate graph showing temperature trends for the time period. A major period of flood basalt eruptions, with at least a million cubic kilometers of lava produced. A major meteorite impact, with a crater diameter greater than 100 km. For more details, see below. T This column represents the Earth's temperature, with dark blue representing an ice age. O₂ This column represents the percentage of oxygen. Darker blue means higher oxygen levels. See below.
Era/Period/Epoch			T	O₂	Time (Myr ago)
Hadean era (origin of Earth/Moon system)				0	4600-4000
Archean era (first life: anaerobic bacteria)				0	4000-2500
PaleoProterozoic era (aerobic bacteria, oxygenation of atmosphere)				5?	2500-1600
MesoProterozoic era (first Eukaryotic cells)				9?	1600-805
Neoproterozoic era	Sturtian period (first "snowball earth" event)			16	805-605
Neoproterozoic era	Ediacaran (or Vendian) period (soft-bodied multicellular animals)			17	605-543
Paleozoic era (First animal and plant life)	Cambrian period (marine animals with hard body parts and eyes)			18	543-490
	Ordovician period (first jawless fish, algae)			16	490-443
	Silurian period (fish with jaws, insects, land plants)			20	443-417
	Devonian period (armored fish, large forests)			25	417-354
	Carboniferous period (amphibians, forests and swamps)			30	354-290
	Permian period (origin of mammals and reptiles) (?)			20	290-252
Mesozoic era (Age of Reptiles)	Triassic period (amphibians, reptiles, conifers)			15	252-200
	Jurassic period (dinosaurs, first birds)			13	200-135
	Cretaceous period (flowering plants, dinosaurs)			18	135-65
Cenozoic era (Age of Mammals)	Tertiary period	Paleocene epoch (grass, mammal diversification)		17	65-55
		Eocene epoch (angiosperm herbs, trees with fruit)		18	55-38
		Oligocene epoch (elephants, horses)		19	38-26
		Miocene epoch (kelp forests, grasslands)		20	23-6
		Pliocene epoch (diversification of apes and hominids)		21	6-1.8
	Quaternary period	Pleistocene epoch (ice ages, human ancestors)		21	1.8-0.01
	Quaternary period	Holocene epoch (agriculture, civilisation)		21	0.01-0
	The second half of the Age of Animals 50: 150: 250:				0-500
Cenoproterozoic (plants, animals, eukaryotes disappear)					500-1000
Cenoarchean (Earth is again only fit for bacteria)					1000-5000
Neohadean (Sun expands to reach Earth's orbit, life can no longer exist)					5000+

Detailed Evolutionary Time line
Time (Myr ago)	Evolutionary Step
Hadean Era
4600	The solar system is formed from the debris of several supernova explosions. Planets condense from the solar system nebula.
4500	A large body about the size of Mars collides with Earth. Most of the crust is ejected into space, congealing into the moon. The reduced amount of crust on Earth results in thin crustal plates that can subduct (slide) under each other, making plate tectonics possible. This body may have formed at Lagrange points (details) on the Earth's orbit.
4400	Outgassing of volatiles from the mantle to the atmosphere. Abundant impact cratering.
4300	Oldest known mineral crystals. Possible existence of first continents.
4200	Early oceans form.
Archean Era - The First Life (Anaerobic Bacteria)
4000	Beginning of crust formation and start of tectonic activity.
3900	Theoretical origin of the first anaerobic eubacteria. According to Carl Woese [ref], archaebacteria originate at the same time.
3800	Oldest known sedimentary rocks in Isua, Greenland, with banded iron formations and isotopic evidence for life. However, this interpretation is disputed.
3700	Appearance of banded iron formations. Iron dissolved in the ocean was oxidized and precipitated, probably by bacteria. The oxygen itself was produced by photosynthetic bacteria. This process continued for nearly two billion years, and is the source of all iron mined today. There is 20 times more oxygen in tied up in banded iron formations than in the present atmosphere.
3465	Fossil stromatolites in the Apex chert of northwest Australia, actually dated to within 10 million years. These microbial mats imply widespread photosynthetic bacterial communities. Given the existence of banded iron formations, the bacteria were probably cyanobacteria.
2800	Large continents formed from raised portion of plates known as the "Precambrian shields".
Proterozoic Era - Aerobic bacteria, oxygenation of the atmosphere
2500	Peak of banded iron formations. Oxygen production reaches the point where it is no longer all absorbed by elements such as iron. It begins to enter the atmosphere and steadily works to strengthen the ozone layer and change the Earth's chemically reducing atmosphere into a chemically oxidizing one.
2200	Worldwide glaciation event (an early Snowball Earth), possibly caused by cyanobacteria drawing down the carbon dioxide supply.
1900	First appearance of Grypania, spiral fossils several cm long. The could be either eukaryotes, or cyanobacterial colonies.
1800	Replacement of banded iron formations by red beds (oxidized iron sediments), indicating worldwide oxygen atmosphere of about 3%, compared to 20% today..
1600	Fossil Acritarchs. These are interpreted as the first eukaroyotes, but may simply be large cyanobacteria.
1400	Appearance of terrestrial cyanobacteria (desert crust)
1200	Bangiomorpha pubescens, from the Hunting Formation, arctic Canada, is the earliest taxonomically resolved eukaryote on record. More importantly, it is the earliest known example of both sexual reproduction and complex multicellularity.
1100	A rapid diversification of planktonic eukaryote assemblages begins, followed by a decline in both abundance and diversity, between 900 and 675 Ma.
1000	Seilacher et al. (1998, p81) reports trace fossils from the one billion year old Chorhat Sandstone formation in the Son Valley, central India, which the authors interpret as the burrows of triploblastic undermat miners - infaunal animals that excavated tunnels underneath microbial mats “which served as a food source as well as an oxygen mask for the worm like animals that were exploiting its decaying base”.
NeoProterozoic Era - Multicellular life
760	The supercontinent Rhodinia splits apart, creating more coastline with continental shelves hospitable to plankton. Photosynthesis removes carbon dioxide from air, reducing greenhouse effect and cooling the Earth.
750-720	The Sturtian Ice Ages. (Snowball Earth, [WAL1]). One view has it that the entire ocean froze to a depth of 1.5 km at the equator and 3 km at the poles, with an average world wide temperature of -40^o C. These ice ages last millions of years, until the buildup of carbon dioxide from volcanoes heats the atmosphere enough to melt the ice, leading to average temperatures of +40^o C. The alternative Slushball Earth hypothesis suggests these ice ages were less severe.
630	Varanger or Marinoan glaciation. The second "Snowball Earth" event.
580	Gaskiers Glaciation. The iron content of deep-sea sediments shows that the deep ocean was anoxic (no oxygen) and ferruginous (iron rich) before and during the Gaskiers glaciation 580 million years ago and that it became oxygenated afterward. The first known members of the Ediacara biota (soft-bodied multi-cellular animals) arose shortly after the Gaskiers glaciation, suggesting a causal link between their evolution and this oxygenation event. A prolonged stable oxic environment may have permitted the emergence of bilateral motile animals some 25 million years later. [Science Sep 07]
578	A meteor similar in size as the one at the K/T boundary hits Australia to form the Acraman crater. An increase in biological complexity follows immediately after, possible stimulated by recovery from mass extinction of plankton.
Paleozoic Era - The first plant and animal life
543	The "Cambrian Explosion" results in the rapid evolution of animals with hard body parts. These are easily fossilized, giving the impression that life suddenly appeared. Their Precambrian ancestors were much smaller, and not easily preserved as fossils. The "explosion" may be more a rapid increase in size rather than change in structure. Hard body parts such as shells evolved to support the the larger size of the organisms. The segmented shape of many arthropods increases the surface area available for respiration, and may be an adaptation to the low oxygen levels at the time.
510	The first fish have a cartilaginous rather than a bony skeleton and lacked jaws, simply having a hole for a mouth. But they have four "limbs" (paired fins) used for propulsion and navigation in the water. These fish tended to have thick bony plates as armor.
485	The earliest evidence for terrestrial activity by animals, provided by trace fossils, are tracks made by multiple ~50 cm-sized, many legged animals preserved in an eolian sandstone in the Nepean Formation (Potsdam Group) near Kingston, Ontario. These track-makers were probably amphibious arthropods. This seems to precede the existence of plants on land.
490	Cambrian period ends with a mass extinction, probably caused by an ice age. This involved the loss of 75% of the trilobite families, half the sponge families, and many brachiopods and snails. Ordovician period begins.
465	The Great Ordovician Diversification Event: Life's largest burst of evolutionary variety involving marine organisms such as the brachiopods, shellfish living on the sea floor. This event, which took place over 20 million years, coincides with a large increase in meteor impacts caused by an asteroid collision. The exact connection between meteor impact and this diversivication is not yet known. [Science: Dec 21, 2007]
458	Fossils of spores and banded tubes indicate the first presence of land plants (Bryophytes) in the late Ordovician. These plants evolved from green algae.
450-440	Ordovician period ends with the second most severe mass extinction, accompanied by a glaciation. One [crazy] theory claims it was caused by a nearby gamma ray burst. This would destroy the ozone layer, and shroud the earth with nitrous oxide smog. The sun's ultraviolet radiation would kill life near the surface, and the smog would trigger the observed ice age, causing further extinction.
430	During the Silurian period, organisms such as liverworts, lichens, fungi and mosslike plants inhabited humid nearshore habitats. The first land animals were scorpions. By the end of the period, the evolution of true vascular plants, with water conducting tissues and pores for gas exchange, had begun.
420	The Devonian period begins with oxygen levels one half present day levels, and ten times the carbon dioxide. The first land animals, primitive wingless insects, appear. There were two major continents, Laurussia in the north and Gondwana in the south, gradually approaching each other, closing the Iapetus Ocean between them.
385	In the second half of the Devonian, oxygen levels were rising. Primitive plants diversified into four major lineages: the seedless Lycopods, Horsetails and Ferns, plus the seed-bearing Gymnosperms. The Devonian is known as the Age of Fishes, and all major groups of fish were present. Aided by the high oxygen levels, a second wave of animals began to colonize the land.
380	The first amphibians move onto land. Evolving from fish, their fins develop into legs. They still must return to the water to lay their eggs.
360-354	Devonian period ends with a sequence of mass extinctions, possibly caused by newly evolved plants drawing down carbon dioxide triggering an ice age. Oxygen in the atmosphere had reached near present levels.
330	Further evolution of gymnosperms (coniferous trees) in the Carboniferous period, which are distinguished by their seeds. This is a more effective means of reproduction, as the seeds can survive in a dry environment. These softwood trees are still with us today. As forests were converted to coal, carbon dioxide levels fell to today's levels, and oxygen levels rose to above 30% (compared to 21% today). This encouraged the development of flying insects, some of which had wingspans of up to three feet.
320	The first (Anapsid) reptiles evolved from amphibians, distinguished by their eggs that could be laid and hatched outside of water. Like gymnosperm seeds, these may be an adaptation to a drier environment.
300	The Permian period sees the original Anapsid reptiles diverge into the Synapsids and Dyapsids. The Synapsids gave rise to the Therapsids, also known as mammal-like reptiles, the ancestors of today's mammals. By the late Permian these animals were the first to achieve endothermy (warm-bloodedness). The Dyapsids are the ancestors of reptiles such as snakes and Dinosaurs.
285	Origin of the order Coleoptera, or beetles, which now represent one fourth of all names species. [Science: Dec 21 2007]
252	Permian period ends with the greatest mass extinction ever, killing 90-95% of both land and sea species [BEN1]. This was accompanied by rapidly falling oxygen levels. Multiple causes include the formation of the super-continent Pangea, glaciation at the poles, and massive flood basalt eruptions in Siberia. Anoxic oceans released large amounts of toxic hygrogen sulfide. Much less likely causes: a meteor impact (Bedout, north of Australia), and even clumps of dark matter.
Mesozoic era - The Age of Reptiles
250	The Triassic period begins with the devastated ecosystem left by the Permian extinction. On land a single species, Lystrosaurus (a 1 meter long pig-like herbivore) represents 95% of large animal life.
224	The first dinosaurs appear. They are small, but being bipedal gives them a speed advantage. The warm climate of the Mesozoic era gives cold-blooded reptiles a relative advantage over warm-blooded mammals. Their efficient respirotory system is well adapted to the low oxygen levels at the time.
200	The Triassic era ends with a mass extinction of about 70% of all species. The extinction on land began about 700,000 years before the extinction in the ocean. This was accompanied by a rise in carbon dioxide levels and massive volcanic activity.
210-160	The second generation of dinosaurs appear in the Jurassic. Mostly herbivores, they included the largest dinosaurs of all time. There was also a significant flood basalt eruption.
145	Archaeopteryx, possibly the ancestor of modern birds, was a feathered dinosaur capable of flight..
130	Origin of flowering plants (angiosperms). These evolved to become today's dominant vegetation, including all grasses, hardwood trees, all crops, etc.
93	During the Cenomanian-Turonian mass extinction the entire ocean became statified and stagnant. The deep ocean became anoxic and all life in it died. This date has also been identified as the origin of the common ancestor of the grasses that exist today [Science Nov 2008].
80	The most prominent plant-eating dinosaurs were digesting different varieties of grass between 65 million and 71 million years ago, But grasses must have originated considerably earlier, well over 80 million years ago, for such a wide variety to have evolved and spread to the Indian subcontinent.
75	The third generation of dinosaurs reaches its peak in the Cretaceous, then starts to slowly decline. These include most of the dinosaurs featured in "Jurassic" Park. The climate of the Cretaceous was very warm, with large portions of the continents flooded by sea levels up to 100m (330 feet) above today's levels. Carbon dioxide levels were four to eight times higher than today. Average temperatures were about the same everywhere on Earth (about 10° C higher than today), from the poles to the equator. Dinosaurs and semi-tropical vegetation are known from within 10° of the Cretaceous poles.
70-65	The climate cools and the extinction rate of dinosaurs increases. Around 66 Mya massive flood basalts occurred in the Deccan Traps in India, releasing as much energy as the large Chicxulub Meteorite 65 Mya. A recent theory has it that the heat of re-entry of impact ejecta killed all land animals not burrowed into the earth. There were a lot of extinctions at sea, including the ammonites. Both events triggered the release of carbon dioxide, leading to global warming in the early Paleocene.
Cenozoic era - The Age of Mammals
60	In the Paleocene, birds began to diversify and occupy new niches. Also, the first appearance of modern pines, cacti and palms. Also, a flood basalt event.
58	First fossil evidence of dry land grass. Fossil pollen has been found in South America and Africa. Although it may appear to be primitive, grass is an evolutionary latecomer, arising in the late Cretaceous.
55	Paleocene-Eocene Thermal Maximum. An episode of rapid and intense warming (up to 7°C at high latitudes) which lasted less than 100,000 years. This occurred at the same time as a flood basalt eruptionthat occurred when the North Atlantic ocean opened over the ancestral Iceland hot spot. The temperature rise was caused by greenhouse gas release during magma interaction with basin-filling carbon-rich sedimentary rocks. A combination of ocean acidification from rapidly rising carbon dioxide levels, and ocean anoxia from reduced circulation led to a mass extinction of deep ocean life. Other theories claim the warming was caused by the release of methane hydrates, or it was triggered by comet impact.
50	Warm global climate in the Eocene. Subtropical rainforests with very broad leaves at the poles. Primitive monkeys have evolved. Appearance of the first whales and horses (Eohippus). Around this time the continent of India began to collide with Asia, causing the rise of the Himalaya Mountains and the Tibetan Plateau. This led to a change in weather patterns and an increase of removal of carbon dioxide via weathering, which may have caused the climate to begin to cool [Science Aug 2008].
35	Two meteorite impacts create 100 km craters at Chesapeake (USA) and Popigai (Siberia), but there is no evidence of linked extinctions. Antarctica separates from South America (starting 41 Mya [Science 21 April 2006]) and Australia (35 Mya), allowing the formation of a circumpolar current. This causes the climate to become cooler and drier, and an ice cap starts forming on Antarctica. The Arctic ocean remains unfrozen.
32	The origin of the C4 photosynthesis pathway in grass, at a time of declining carbon dioxide levels, which fixes carbon to form carbohydrates at higher temperatures and lower CO₂ levels [Science Nov 2008].
25	Around this time grasses begin to expand and diversify. Almost all our agriculture depends on it, for grazing animals and cereal crops such as wheat and rice.
20-12	The chimpanzee and hominid lines evolve.
4.6	The relatively warm conditions during the Pliocene come to an end as North and South America become joined at Panama. This joining was brought about by a shift of the Caribbean plate, which moved slightly eastwards and formed a land bridge across the Isthmus of Panama. The formerly separate land ecosystems merge, leading to minor extinctions, while the Pacific and Caribbean marine ecosystems begin to diverge. The Atlantic Ocean, now cut off from the Pacific, develops a complex current system that leads to increased snowfall in North America, which accumulates into ice sheets, starting the current ice age. The tectonic plates of India and Asia also collided, which formed the Himalayan Mountains. Generally the climate of the Pliocene is thought to have been warmer than it is today. The warmest phase was in the middle of the epoch, the interval between 3 and 4 million years ago. The climate was especially mild at high latitudes and certain species of both plants and animals existed several hundred kilometers north of where their nearest relatives presently exist. Carbon dioxide levels were around 380 ppm, similar to today, while temperatures were 3° C higher. Less ice at the poles also resulted in a sea level that is thought to have been about 25 meters higher than it is today.
2-0.01	The current ice age begins, with cycles of roughly 100,000 years of ice followed by about 10,000 to 15,000 years of a warm inter-glacial period.
0.01	The current inter-glacial period (the so-called Holocene epoch) begins.
The Future - Much like the past in reverse (WAR1)
0-?	The present interglacial period is due to end soon. Carbon dioxide released by burning fossil fuels may introduce climate instability, and/or may temporarily delay the return of the ice age.
?-100	Fossil fuels are exhausted, carbon dioxide is absorbed by vegetation and weathering. The age of glaciation returns.
100-500	Increasing warmth from the sun puts and end to glaciation. Carbon dioxide continues to be absorbed until levels become too low to support plant life. The continents begin to coalesce, forming a new Pangea Ultima, creating conditions similar to those at the time of the Permian extinction.
500-1000	Plant life dies from ever increasing temperatures and lack of carbon dioxide. Animal life dies on land, becoming confined to the sea.
1000-5000	Animal life ends, and bacterial life is confined to the oceans, until they are evaporated by rising temperatures.
5000+	The sun enters its red giant phase, expanding until it reaches Earth's orbit. Life may persist on moons in the outer solar system until the sun dies out.

Variations in Carbon Dioxide and Oxygen in the Atmosphere and Ocean

The two charts below show how the amount of carbon dioxide and oxygen in the atmosphere has varied significantly over the past half billion years. The dark blue bars on the upper chart (from IPCC 2007) show the extent of ice ages over this time period. According to Peter Ward, in his book Out of Thin Air, changing oxygen levels (shown in the lower chart from Science) have been the driving force in evolutionary change. The major mass extinctions coincide with low oxygen levels. But [Science: Aug 2008] points out that charcoal is found in the low-oxgygen eras, and a concentration of at least 15% oxygen is required for that kind of combustion.

Note in particular during the Carbiniferous period 300 millions years ago the low levels of carbon dioxide and ice age conditions, and extremely high oxygen levels.
See here for more details on these charts in the context of climate change.

	Key to the numbered intervals (from Science, 27 April 2007)
(1)	In the Cambrian period, the origin of the first animal body plans coincided with a rapid rise in atmospheric O₂ concentration.
(2)	The late Cambrian / Ordovician lineages of fish and cephalopods evolved anatomical structures that took advantage of their swimming ability to force larger volumes of water across their gill surfaces, which in turn allowed for increased O₂ uptake.
(3)	Diversification of marine animals occurred during the Ordovician rise in O₂.
(4)	The conquest of land by animals occurred during two independent phases of high O₂ concentration. The earliest, during the early Devonian ~410 million years ago, involved mainly arthropods.
(5)	The Late Devonian was the first of three major extinctions that were also followed by an extended period of low atmospheric O₂ concentration. The aftermath of a major extinction is often a time of rapid evolution, potentially producing novel body plans. Many of these new body plans may have supported more efficient respiratory systems, which may have been selected for under low-O₂ regimes that coincide with postextinction time periods.
(6)	A low oxygen period of stasis, known as Romer's Gap, saw very little evolution.
(7)	The second phase of the conquest of land by animals, during a time of high O₂ concentration, involved both arthropods and vertebrates.
(8)	With increasing O₂ concentrations through the Carboniferous and Permian, gigantism developed in several arthropod groups, and body size increased across primitive reptile-like animals and their descendants. The gigantism has classically been attributed to an increase in diffusive capacity caused by an increase in atmospheric O₂ concentration. This may explain the effect seen in egg-laying vertebrates, because diffusion across the eggshell will be increased and have an effect on hatchling and therefore adult body size. Alternatively, in some insects, body size is limited by the amount of their body that can be allocated to trachea. Because tracheal diameters decrease with increased O₂ concentration, a higher maximal body size can be achieved in times of higher O₂ concentration.
(9)	The Permian-Triassic mass extinction, the worst ever, was accompanied by falling O₂ concentrations.
(10)	During the latter part of the Triassic, a time of low modeled O₂ concentrations, the evolution of the dinosaur body plan involved a novel air-sac system which was inherited in modified form by their descendants, the birds. Air-sacs allow highly efficient respiration even at high altitude. They may similarly have conferred a respiratory advantage to early dinosaurs as compared to other contemporary terrestrial animals.
(11)	The Triassic-Jurassic extinction was also a time of low oxygen levels.
(12)	The increase in mammalian body size in the Tertiary has been linked to rising O₂ concentrations

Carbon dioxide through time, as computed with GEOCARB, a computer program for estimating past levels of carbon dioxide. Each of the bullets designate a mass extinction, and they can be seen to correspond either with high or sharply increasing carbon dioxide levels.

From: Peter Ward, Under a Greening Sky, page 135.

Oxygenation of the Deep Ocean

The evolving deep ocean. In the traditional model, oxygenation of the ocean occurred 1.8 billion years ago, with only brief recurrence of ferruginous conditions during later global glaciations (11). Canfield et al. now suggest that widespread deep-ocean anoxia lasted to 540 million years ago and perhaps a little longer. Instead of early oxygenation, after 1.8 billion years ago the deep ocean was dominated by H₂S, followed by a repeat of widespread iron conditions (1).

References: Books
Natural History Overview
WAR1	The Life and Death of Planet Earth, by Peter D. Ward and Donald Brownlee, 2003. This book presents the case (known as the Rare Earth Hypothesis) that life on Earth is dependant on a number of unusual events, and that similar planets are likely to be rare throughout the galaxy. Of particular interest is the theory that plate tectonics, vital in many ways to keeping the Earth habitable, was made possible only by the impact on the young Earth that formed the Moon. The projections into the future on this page are based on this book.
HAZ1	Genesis, the Scientific Quest for Life's Origins, by Robert M. Hazen, 2005. Hazen introduces the theory of emergence, where complexity emerges from simpler elements when energy is applied. He then examines the possible orgins of life from the perspective of chemistry, geology and astrophysics.
MAR1	What is Life?, by Lynn Margulis and Dorian Sagan, 2000. An introduction to the history of life on Earth, with particular attention to the microbiology of the Precambrian Era.
GOU1	The Book of Life, edited by Stephen Jay Gould, 2001. A history of the evolution of life.
FOR1	Life: a Natural History of the First Four Billion Years of Life on Earth, by Richard Fortey, 1997. A very readable summary of the history of life on Earth, written by a paleontologist. The first chapter on the origin of life presents as fact that the first bacteria were extremophile archaea, which is only one of many hypotheses, but the rest of the book is excellent.
FOR3	The Earth, An Intimate History, by Richard Fortey, 2004. An account of the geology of the Earth, told from the perspective of his personal travel to various locations around the world of geological interest.
WAR4	Under a Green Sky, by Peter Ward, 2007. This book is an attempt to bridge the "canyon" that exists between climate scientists for whom paleoclimate is the study of the Pleistocine ice ages, and paleontologists who investigate the major mass extinctions that have afflicted the earth many times in the more distant past. This subject has been dominated in recent decades by hypothesis that the end-Cretaceous extinction was caused by a massive meteorite strike. After this was established to be fact, the search was on for the evidence of the impacts that caused the other mass extinctions. But no convincing evidence has been found. Instead, Ward has discovered a patterm to many extinctions involving increased greenhouse gases from flood basalt eruptions leading to ocean anoxia and hydrogen sulfide gas poisining. A parallel is drawn to the global warming in progress today. Unfortunately, in the last three chapters this book degenerates into hysteria riddled with basic scientific errors. Read with caution, or skip it altogether.
Precambrian Origin of Life
SCH1	Cradle of Life - The Discovery of Earth's Earliest Fossils, by J. William Schopf, 1999. An account of the fossil evidence for bacteria and Eukaryotic organisms in the Precambrian era. Schopf supports the early origin of cyanobacteria and oxygenation of the atmosphere. First few chapters here, critical opinion here.
WAL1	Snowball Earth, by Gabrielle Walker, 2003. A very readable account of the development of the theory of worldwide glaciations at the end of the Precambrian era. Also, here is a 2002 paper by Hoffman and Schrag giving a detailed account of this idea. Here is a negative review of the book. OK, so there is a low fact to chatter ratio. Still, it was pleasant to read, and I liked the insight into how geologists actually work. A valid criticism is its bias toward the snowball hypothesis, but the reviewer does not mention that, because he clearly supports the hypothesis as well.
Cambrian Explosion
PAR1	In the Blink of an Eye, by Andrew Parker, 2003. An account of the Cambrian Explosion, based on the idea that it was triggered by the first species (a trilobite) that developed eyes, which enabled effective predation and forced the rapid evolution of itself and all other species. But first see this critical review by Simon Conway Morris, an expert on the Burgess Shale Cambrian fossils. Eyes probably evolved long before the Cambrian, trilobites were not the first Cambrian species, and their calcite eyes are unique.
FOR2	Trilobite! Eyewitness to Evolution, by Richard Fortey, 2000. Although focused on trilobites, chapter five of this book has the best description of the so-called Cambrian Explosion I have seen. In general, a very well written book, highly recommended.
MOR1	The Crucible of Creation - The Burgess Shale and the Rise of Animals, by Simon Conway Morris, Professor of Evolutionary Paleobiology, University of Cambridge, 1999. This book is partly a response to the classic Wonderful Life by Stephen Jay Gould. He takes issue with Gould's hypothesis that complex life began with a large number of phyla (body plans), and disparity has been reduced over time. He also criticizes the idea that the form of today's life is purely a matter of contingency. Read the very detailed review.
Permian / Triassic Boundary Extinction
BEN1	When Life Nearly Died - the Greatest Extinction of All Time, by Michael J. Benton, 2003. An investigation of the Permian mass-extinction. This book presents very recent data that indicates this extinction was more severe both on land and on sea than previously thought.
WAR2	Gorgon: Obsession, Paleontology, and the Greatest Catastrophe on Earth, by Peter Ward, 2004. A first hand account of ten years of investigation into the sequence and causes of the Permian mass extinction. A case for a rapid (50,000 years or less) extinction is methodically presented. The last two chapters then fast forward into a conclusion that de-oxygenation of the oceans and atmosphere was the means of extinction, without nearly the same amount of supporting evidence. Ocean levels dropped, exposing anoxic organic materials to the atmosphere. The newly-exposed materials oxidized, pulling oxygen out of the air, and the iron in these materials rusted, creating the red rock layers that are so distinctive in post-Permian geology. Explosive volcano eruptions from Siberia may have contributed to this loss of oxygen as well, expelling huge amounts of carbon dioxide, carbon monoxide, methane, and other gases into the atmosphere. He also deals critically with the Luann Becker hypothesis that the extinction was caused by a meteor impact.
Cretaceous Tertiary Extinction
COU1	Evolutionary Catastrophes, the Science of Mass Extinctions, by Vincent Courtillot, 1999. He first presents the evidence for the Chicxulub impact being the cause of the K/T extinction. He then develops a theory of how plumes from deep within the mantle cause flood basalt eruptions, and that these are the major cause of mass extinctions.
Evolution
MOR2	Life's Solution, by Simon Conway Morris, 2003. This book has three parts. First, he presents the hypothesis that Earth-like planets are rare in the universe, similar to Ward's Rare Earth. Then he presents a convincing demonstration of how little progress has been made in discovering the origin of life. Finally, most of the book gives many examples of how many aspects of living thing have evolved more than once. The hypothesis is that while many lifeforms are theoretically possible, there are only a limited number of forms that actually work, and evolution tends to converge towards those forms.
WAR3	Out of Thin Air, by Peter Ward, 2006. The thesis of this book, in its own words, is: The history of atmosphereic (and hence oceanic) oxygen levels through time have been the most important factor in determining the nature of animal life on Earth - its morphology and basic body plans, physiology, evolutionary history, and diversity. This hypothesis means that the level of oxygen influenced every large-scale evolutionary adaptation or innovation in the history of animal life on Earth, and that oxygen levels dictated evolutionary originations, extinctions, and the architecture of animal body plans.
NIK1	The Evolutionary Biology of Plants, by Karl J. Niklas, 1997. Comprehensive and rather technical.
Web Sites
	My Related Sites: Paleoclimate provides the history of climate change for the last half billion years. See also the Science of Climate Change for an explanation of global warming issues in the context of Earth's past.
1	The Paleomap Project. This superb site links the changing arrangement of the continents due to plate tectonics with changes in the Earth's vegetation and climate. The Paleogeographic Atlas Project, from the University of Chicago, had detailed maps spanning a shorter period of time.
2	The Tree of Life Web Project. An extensive model of the relationship between all living things, in tree form.
3	Plant Evolution Tour. This site gives detailed information about the evolutionary history of plants.
4	This page was originally inspired by a time line created by Niel Brandt and found at the Talk Origins Archive.
5	Major Events in the History of Life. A more detailed summary of evolutionary history.
6	A detailed examination of changing sea level from the Paleozoic Era (542 to 251 million years ago) is found in [Science Oct 2008].
Meteorite Impacts and Flood Basalt Eruptions
	There are two natural catastrophes that are linked to mass extinctions: Meteorite impacts are well known as the cause of the end-Cretaceous mass extinction, and are linked to some other extinction events as well. Only impacts on land (or continental shelves) persist; impacts into the majority of the Earth's surface covered by ocean quickly disappear. As well, comets and loosely packed meteorites (see the article Are Asteroids Rubble Piles) may not leave craters at all, but may explode in the atmosphere, as happened at Tunguska in Siberia in 1908. Therefore the meteorite record is only partially known. Flood basalts are massive eruptions of lava from deep below the surface. While a large volcano may discharge a few cubic kilometers of lava in its lifetime, the largest flood basalts are measured in millions of cubic kilometers, covering thousands of square miles. Large amounts of dust and gas are released into the atmosphere. Flood basalts can be linked to almost every mass extinction, including the end-Cretaceous event commonly attributed to meteorite impact.

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