Index to this page

The history of life as revealed by the fossil record (with help from molecular phylogenies).

ErasPeriodsEpochsAquatic LifeTerrestrial Life
With approximate dates in millions of years ago in parentheses. Geologic features in green
Cenozoic (66 to present)
The "Age of
Quaternary (2.6 to present)HoloceneHumans in the new world
PleistocenePeriodic glaciationFirst humans
Continental drift continues
Neogene (23–2.6) PlioceneAtmospheric oxygen reaches today's level (21%)Hominids
MioceneAdaptive radiation of birds; continued radiation of mammals
Paleogene (66–23) OligoceneAll modern groups present
Mesozoic (251–66)
The "Age
of Reptiles"
Cretaceous (146–66)Still attached: N. America & N. Europe; Australia & Antarctica; Mass extinction of both aquatic and terrestrial life at the end
Modern bony fishesExtinction of dinosaurs and pterosaurs; first snakes
Extinction of ammonites, plesiosaurs, ichthyosaurs
Africa & S. America begin to drift apart
Jurassic (200–146)Plesiosaurs, ichthyosaurs abundant; first diatoms Archaeopteryx; dinosaurs dominant but mammals (Eutheria) begin to diversify. Adaptive radiation of angiosperms begins toward the end.
Ammonites again abundant
Skates, rays, and bony fishes abundantAdaptive radiation of dinosaurs. Early mammals
Pangaea splits into Laurasia and Gondwana; atmospheric oxygen drops to ~13%
Triassic (251–200)Mass extinctions at the end.Mass extinctions at the end.
Adaptive radiation of reptiles: turtles, lizards and snakes, crocodiles, first dinosaurs, therapsids
Ammonites abundant at first
Adaptive radiation of bony fishes
Paleozoic (542–251)Permian (299–251) Atmospheric oxygen reaches ~30%. Mercury-laden volcanic eruptions killed off 90% of marine species and 70% of terrestrial species at end.
Extinction of trilobitesReptiles abundant. Cycads, conifers, ginkgos
Pennsylvanian (318–299)Warm, humid climate
the Pennsylvanian
and Mississippian
make up the
also called the
"Age of Amphibians"
Great diversity of marine invertebratesFirst reptiles
Coal swamps
Mississippian (359–318) Adaptive radiation of sharksForests of lycopsids, sphenopsids, and seed ferns
First modern amphibians
Adaptive radiation of the insects (Hexapoda)
Atmospheric oxygen begins to rise as organic matter is buried, not respired
Devonian (416–359)
The "Age of Fishes"
Extensive inland seasCartilaginous and bony fishes abundant. Ammonites, nautiloids, ostracoderms, eurypteridsFerns, lycopsids, and sphenopsids
First gymnosperms
Silurian (444–416)Mild climate; inland seas First bony fishesFirst myriapods and chelicerates. First land plants.
Ordovician (488–444)Mild climate, inland seasTrilobites abundant Fungi present
First insects
Cambrian (542–488) First vertebrates (jawless fishes). Eurypterids, crustaceans
mollusks, echinoderms, sponges, cnidarians, annelids, and tunicates present. Trilobites dominant.
No fossils of terrestrial eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present.
Periodic glaciation
Proterozoic (2500–542)Ediacaran
Fossil evidence of multicellular algae, fungi, and bilaterian invertebrates
Evidence of eukaryotes
~1.8 x109 years ago
Archaean (4000–2500)Evidence of microbial life
3.5–3.7 x109 years ago

The Geologic and Evolutionary Record

A remarkable feature of the table above is how often evolutionary changes coincided with geologic changes on the earth. But consider that changes in geology (e.g., mountain formation or lowering of the sea level) cause changes in climate, and together these alter the habitats available for life. Two types of geologic change seem to have had especially dramatic effects on life:

Continental Drift

A body of evidence, both geological and biological, supports the conclusion that 200 million years ago, at the start of the Mesozoic era, all the continents were attached to one another in a single land mass, which has been named Pangaea.

This drawing of Pangaea (adapted from data of R. S. Dietz and J. C. Holden) is based on a computer-generated fit of the continents as they would look if the sea level were lowered by 6000 feet (~1800 meters).

During the Triassic, Pangaea began to break up, first into two major land masses:

The present continents separated at intervals throughout the remainder of the Mesozoic and through the Cenozoic, eventually reaching the positions they have today.

Let us examine some of the evidence.

Shape of the Continents

The east coast of South America and the west coast of Africa and are strikingly complementary. This is even more dramatic when one tries to fit the continents together using the boundaries of the continental slopes, e.g., 6000 feet (~1800 meters) down, rather than the shorelines.



External Link
View an animation of the breakup of Pangaea from the Jurassic (200 million years ago) to the present.
Please let me know by e-mail if you find a broken link in my pages.)

The Impact Hypothesis

The Cretaceous period, the last period of the Mesozoic, marked the end of the Age of Reptiles. It was followed by the Cenozoic era, the Age of Mammals. Although extinctions have occurred throughout the history of life, an extraordinary number of them occurred in a relatively brief period at the end of the Cretaceous. Why?

The Alvarez Theory

Louis Alvarez, his son Walter, and their colleagues proposed that a giant asteroid or comet striking the earth some 66 million years ago caused the massive die-off at the end of the Cretaceous. Presumably, the impact generated so much dust and gases that skies were darkened all over the earth, photosynthesis declined, and worldwide temperatures dropped. The outcome was that as many as 75% of all species — including all dinosaurs — became extinct.

The key piece of evidence for the Alvarez hypothesis was the finding of thin deposits of clay containing the element iridium at the interface between the rocks of the Cretaceous and those of the Paleogene period (called the K-Pg boundary after the German word for Cretaceous). Iridium is a rare element on earth (although often discharged from volcanoes), but occurs in certain meteorites at concentrations thousands of times greater than in the earth's crust.

After languishing for many years, the Alvarez theory gained strong support from the discovery in the 1990s of the remains of a huge (180 km in diameter) crater in the Yucatan Peninsula that dated to 66 million years ago.

The abundance of sulfate-containing rock in the region suggests that the impact generated enormous amounts of sulfur dioxide (SO2), which later returned to earth as a bath of acid rain.

A smaller crater in Iowa, formed at the same time, many have contributed to the devastation. Perhaps during this period the earth passed through a swarm of asteroids or a comet and the repeated impacts made the earth uninhabitable for so many creatures of the Mesozoic.

This time in the earth's history was also marked by periods of intense volcanic eruptions which could have contributed to the extinctions.

Other Impacts?

A mass extinction of non-dinosaur reptiles occurred earlier, at the end of the Triassic. It was followed by a great expansion in the diversity of dinosaurs. The recent discovery of a layer enriched in iridium in rocks formed at the boundary between the Triassic and Jurassic suggests that impact from an asteroid or comet may have been responsible then just as it was at the K-Pg boundary.

The largest extinction of all time occurred still earlier at the end of the Permian period. There is evidence off the coast of Australia that a huge impact there may have contributed to the extinctions at the Permian-Triassic (P-T) boundary.

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16 February 2023