For decades the origin and evolution of life was restricted to the fossil record that recorded hard-shelled life. We now know, through determination of absolute ages by radioactive decay, that this record only record the last 500 m.y. or so of life. Prior to that, life existed as soft-bodied organisms, or even earlier, as single cell bacteria (prokaryotes) or single-celled organisms with nuclei (eukaryotes).
Click on the thumbnails below for larger image.
Above is a diagram that casts the origin and evolution of life into a 24 hour clock.
The oldest microfossils, composed of single-celled organisms that probably were similar to cyanobacteria, are 3.5 b.y. old, and are found in Western Australia (not the same locality where the very old zircon mineral grains were found). These fossils were reported in 1993 (above). Yes, they are pretty fuzzy and perhaps not convincing! This continues to be debated.
More convincing evidence for life in the Archean comes from fossil layered microbial communities called stromatolites (above). Although the 3.5 b.y. old microfossils are still debated, people pretty much agree that the fossil record for life is undisputable by about 3.0 b.y., and stromatolites are part of this evidence.
Fossil bacteria are universally accepted for the Proterozoic (above), where the images (and chemical compositions) are much more clear than the fuzzy images for the 3.5 b.y. old microfossils above.
The Proterozoic microfossils are much more similar to the modern cyanobacteria in the image above. The occurrence of cyanobacteria early in earth's history is critical, since their metabolic "waste product" is oxygen, and it was essential to produce high levels of oxygen in the earth's atmosphere before more complex life (which requires different means of metabolism and energy storage) could evolve.
In the latest part of the Proterozoic (~ 600 m.y. ago), multi-cellular, complex life is recorded in the fossil record.
Just before the Cambrian, in the Late Proterozoic, large, multi-cellular life that had soft bodies (jellyfish, etc.) are found in the fossil record (above).
The base of the Cambrian is defined as the occurrence of the first-hard shelled fossils. This does not mean that complex life did not exist before the Cambrian, just that the fossil record is much more unlikely to record soft-bodied life. One of the most common fossils in the Cambrian is the trilobite (Wisconsin state fossil), which are shown here (above).
Artist drawing of the bottom of the Cambrian shallow sea floor, showing trilobites (imagine these crawling around on the Cambrian sea floor at Devil's Lake state park 550 m.y. ago!) (above).
Ordovician life continued to increase in complexity from that of the Cambrian, and appearance of extensive coral reefs occurred. This was accompanied by extensive limestone formation, such as is well shown by the Ordovician limestone rocks that can be found surrounding Madison (look at the road outcrops next time!).
Ordovician life included a larger diversity of predators, including straight-shelled "nautiloid-like" or "squid-like" creatures shown here.
Another example of the diverse Ordovician life in the shallow inland seas of the continents. Note the complex flora (plants) and fauna (animals).
Marine life in the Silurian continued much like the Ordovician, with abundant corals, bivalves (clam-like animals), trilobites, and nautilus-like predators (above).
A major change, however, in Silurian life was the appearance of fish (above).
Devonian life in the shallow inland seas continued much as it had in the Silurian, with extensive coral reefs and ocean plants (above).
Another artist's rendering of Devonian marine life (above).
The major change in life in the Devonian occurred not in the oceans but on land. Fr most of Earth's history, including the early part of the Paleozoic, the land was devoid of plant life - it must have looked quite desolate. However, land plants became common on land in the Devonian, paving the way for life to move out of the oceans and on to the land. The first amphibians are found in the Devonian.
Fossil evidence for the first land animals is quite abundant in the Devonian. The top photo shows the head and poison claw of a centipede. The middle photo shows a fossil compound eye of an insect, from the Devonian. The bottom photo shows a millipede (above image).
Mississippian marine life was marked by extensive shallow reef communities, including flora such as crinoids (above).
With much more land surface exposed in the Pennsylvanian time, as compared to the Mississippian time, extensive tropical forests were formed. These would eventually be fossilized into extensive coal deposits in Pennsylvania as well as other parts of the world (above).
Shallow marine Permian life included extensive reefs, including sponges (shown) and clam-like animals (above).
In the Permian on land, reptile-like creatures became quite large, paving the way for the dinosaurs that were to dominate the Mesozoic (above).
However, at the end of the Permian, 245 m.y. ago, the largest mass extinction of life on Earth occurred; over 90% of the species on Earth went extinct. The most likely culprit was very large volcanic eruptions in several parts of the world (the largest being in Siberia), which polluted the atmosphere with dust and acid rain.
Of course, the most famous life of the Mesozoic are the dinosaurs. Based on hip structure, there are two branches. The saurischian ("lizard hipped") dinosaurs have a pelvis where the pubic bone pointed forward and downward (the right side branch). The saurischians included the carnivorous theropod dinosaurs such as Tyrannosaurus Rex, and the huge plant-eating sauropods. The ornithischians ("bird-hipped"), which were all herbivores, are shown on the left group (above).
Although the dinosaurs claimed all the limelight in the Mesozoic, mammals were present since the Triassic as small rodents (above).
Jurassic dinosaurs included the famous herbivore stegosaurus and the carnivore Allosaurus. Note that despite Steven Spielberg's movie title, Tyrannosaurus Rex did not exist in the Jurassic (above).
The diversity of life in the Cretaceous was unparallel, including a large number of large marine animals, as well as flying dinosaurs (above).
The classic dinosaur paintings are from the Cretaceous, in large part because this was when the famed Tyrannosaurus Rex lived. Note how in the background a volcano erupts, adding to the drama of the scene (it makes it look more "prehistoric"), but supported by the fact that enormous amounts of volcanism was occurring in the Sierra Nevada from the subduction zone that lay off the west coast of North America in the Mesozoic. However, such a scene would not be appropriate, for example, in Europe or the Middle East, where there were no Cretaceous subduction zones but plentiful Tyrannosaurus Rex and other dinosaurs. The late Cretaceous was one of the warmest climates the Earth has seen, which undoubtedly promoted growth of very large creatures (above).
However, at the end of the Cretaceous, 65 m.y., saw a catastrophic impact of an asteroid in the Yucatan Peninsula (eastern Mexico), which generated such dust in the atmosphere that sunlight was reduced, and the temperature plummeted. This caused almost all dinosaurs to become extinct (although some branches survived as birds). The colder climate of the Cenozoic was perfect for mammals, who thrived and came to dominate the Earth in the Cenozoic.
In the later part of the Cenozoic, in the Quaternary period, continental ice sheets covered many parts of the continents, including the Upper Midwest. The latest of these glacial stages, which ended about 10,000 years ago, is called the Wisconsin Glaciation because the evidence is so well preserved in our state.
Spring 2003 Semester