Time Line of Evolution:
This is a highly simplified description of how life is envisioned to have evolved on earth. For a good and entertaining introduction to this topic, I refer you to Richard Fortey’s book LIFE.
PRECAMBRIAN ERA:
4.650,000,000 years ago:
FORMATION OF THE EARTH—violent accrual of debris and meteorites form the earth. Moon probably split off due to a particularly violent collision with a meteorite or comet. Radioactive decay of elements in the interior of the earth produce a molten core which allows the earth to be warmer than it might otherwise have been. Meteorites also contribute many of the elements necessary for life, including carbon (from carbonaceous chondrites) and iron.
Between the formation of the earth and the appearance of the first recognizable cells, life must have begun in an, as yet, unknown way. The fact that all life on earth share the same basic nucleic acid and amino acid building blocks, that all common amino acids used by living beings on earth are the same rotational orientation, and that all life uses essentially the same genetic code of nucleic acid residues to code for amino acids all indicate that life either came into existence only once on earth, or, if it came into existence more than once, the descendants of only one such event ever survived to make up the current biosphere. The possible pathway from a sterile earth to life is unclear but the essential building blocks are easily formed from simple organic compounds and energy such as lightning. Many complex molecules, including amino acids, can be formed essentially spontaneously. How these early, complex organic compounds then took on organization and became self-perpetuating (necessary characteristics for life) is almost completely unknown, though some theories have been put forward involving crystal structures and clays. One intermediate form of life may well have been based on the nucleic acid RNA since this is the only molecule known that can carry out both informational (genetic) and catalytic (enzymatic)_functions.
3,500,000,000 years ago:
PROCARYOTIC CELLS—The simplest kind of living cells are the procaryotes. These cells, unlike our own, have no internal membranous divisions within the cell (i.e. no nucleus, mitochondria, choroplasts, etc.) All activities that in our cells would take place in such membrane bound compartments either are absent or take place in the cell membrane of a procaryotic cell. Procaryotes include two kinds or organisms—the common bacteria that we are all used to (Salmonella, E. Coli, Streptococcus, the plague bacillus, etc.) and the Archaebacteria. The Archaebacteria may have been the first living cells on earth since they can exist under conditions that no other living thing can survive (e.g. boiling temperatures, high acid, high UV light…)_and often derive their energy from simple sources (e.g. sulfur hot springs and undersea vents). So the first living things may have depended on sulfur metabolism and could survive the very harsh conditions that existed on the early earth.
The common bacteria probably evolved after the Archaebacteria. The earliest true bacteria (Eubacteria) were probably the blue-green (cyano) bacteria. These organisms, next to the Archaebacteria, are the most independent organisms on earth and all other life may well be dependent on the cyanobacteria for fixation of Nitrogen from the atmosphere and formation of oxygen from carbon dioxide. The cyanobacteria probably were the first organisms to carry out these two processes. The formation of oxygen by cyanobacteria after 3,500,000,000 years ago changed the entire atmosphere of the earth, produced the ozone layer necessary for terrestrial life, and produced the conditions that we require to live. Prior to the cyanobacteria, the atmosphere would have been unbreathable. Other bacteria evolved after the cyanobacteria.
1,500,000,000 years ago:
EUCARYOTIC CELLS—Eventually more complicated types of cells with internal membrane-bound compartments (organelles) such as nuclei, mitochondria and chloroplasts formed. Such cells, which include our own as well as those of amoebae, fungi, algae, etc, are called eucaryotic. There is strong evidence that eucaryotic cells formed by the cooperation of different procaryotic cells which eventually became so dependent on one another that they became inseparable. During this period cells similar to amoebae, euglena and paramicia may have existed along with the procaryotes. All such life existed in the sea. Nothing lived on land.
700,000,000 years ago:
MULTICELLULAR LIFE—The first multicellular organisms were probably algae which can be found in the fossil evidence from this time. However, many other organisms very different from existing species were common during this period forming a group called the Ediacaran fauna. These were soft-bodied and had either radial symmetry (like a jellyfish) or bilateral symmetry (like a worm or like us). Many of the Ediacaran organisms are very much like modern jellyfish, though they differ in their details. Many are probably unrelated to anything still alive and represent a vastly different phase of evolution that may not have survived the Precambrian.
The Precambrian ended in a way that may well have been as dramatic as the extinction of the dinosaurs. Around 570,000,000 years ago, the Ediacaran fauna disappear, never again to reappear. Some Precambrian organisms continue in the fossil record, but mostly an entirely new set of organisms take over. What caused this mass extinction is not known.
PALEOZOIC ERA:
570,000,000 years ago: CAMBRIAN
During the Cambrian some threshold had been reached. Not only do many new organisms appear, but entire new classes (called phyla) of organisms, including all those in existence today except one—Chordata, which we belong to—originate in the Cambrian period. Possibly atmospheric oxygen reached a critical level or the ozone layer had begun to form or some other threshold had been reached, but either way life that might be familiar to us now came about. This includes worms, sponges and the shelled animals such as mollusks. The oceans teemed with these animals as well as with algae and procaryotes from earlier times that had evolved to live in the new conditions. One group of organisms that no longer exists but was very common throughout the Paleozoic were the trilobites who also come into being in the Cambrian. Trilobites may have had the first "eyes" in the history of life on earth.
A now classic description of the Cambrian explosion of life can be found in Stephen Jay Gould’s Wonderful Life .
500,000,000 years ago: ORDOVICIAN
During this period our own phylum, the youngest one, called Chordata, is first found in the fossil record. The simple, worm-like early Chordates quickly evolved into simple forms of fish (those without jaws, like the lamprey). Some very primitive plants (like liverworts) and millipede-like animals may have appeared on land at this time (the first life to exist on land) but mostly life was still confined to the ocean. The early reefs of the Cambrian probably mostly consisted of sponges and the like. But now true corals were added to the reef ecosystems.
The Ordovician ended with a mass extinction probably associated with an ice age. Many Ordovician species died out, but many continued into the next period.
430,000,000 years ago: SILURIAN
By now the ozone layer had fully formed, protecting the earth from UV radiation and allowing the colonization of land. Consequently, this is when terrestrial life took off from its simple beginnings. Vascular plants (those which can essentially draw water up from the ground through its stem to the leaves via vascular tissues) as well as scorpion-like organisms spread across the land, joining the liverworts and millipedes. In the seas jawed fish appear in the fossil record. Corals continued to spread and trilobites still flourished.
395,000,000 years ago: DEVONIAN
Through the Silurian, life on land would have seemed strange and sparse to us. But now woody plants appear and spread across the land. Simply insects, much like dragonflies fill the air and amphibians (salamander-like rather than frog-like) scramble in the mud. In the seas sharks can now be found, looking much like those we know today, though the exact species would be different.
345,000,000 years ago: CARBONIFEROUS
The early period of the Carboniferous (up to 320,000,000 years ago) is called the Mississippian period and was very hot and wet. Plant life consisted of mostly mosses and fern forests of incredible richness. Amphibians and insects abounded, often taking on large sizes compared with modern versions. From these fern forests, covered up by the mud of later times, formed the coal deposits that fueled the industrial revolution and the oil deposits we are still too dependent upon.
The later period of the Carboniferous (after 320,000,000 years ago) is called the Pennsylvanian period. During this time, the earth cooled considerably and dried out, reducing the fern forests. At this time, perhaps as adaptations to the cooler, drier environment, reptiles appear as well as more familiar plants called gymnosperms (e.g. pine trees). Although this period may not have been as rich as the jungle-like, fern-dominated Mississippian period, it did presage the great era of the dinosaurs.
280,000,000 years ago: PERMIAN
With the Permian we enter a great ice age when many species died out, but we also enter the Age of Reptiles. Reptiles, many crocodile-like, spread widely during this time, including the group of reptiles that eventually gave birth to the mammals (the synapsids). One such synapsid reptile, often mistakenly called a dinosaur (which are diapsid reptiles) that could be found in the Permian was Dimetrodon, the well-known "sail-backed dinosaur." The Permian was also the time when the drifting continents, which had for a long time been drifting towards eachother, finally formed one giant continent called Pangaea. Never again have all the continents formed one land mass, but from the Permian through the Triassic, one could theoretically walk from Alaska, through Europe, to Africa and finally to Australia and Antarctica without ever crossing an ocean.
MESOZOIC ERA:
225,000,000 years ago: TRIASSIC
The Mesozoic, beginning with the Triassic, is the Age of Dinosaurs. It is also the age of the marine reptiles, the plesiosaurs and icthyosaurs, which are not true dinosaurs. It is also the early phase of mammalian evolution. Dinosaurs were modest in the Triassic and few of the familiar dinosaur species had yet appeared. Mammals now existed, though of a primitive sort like shrews. Gymnosperms, first appearing in the Permian, now dominante as cycads (left over from the Permian) and pines.
The great supercontinent, Pangaea, formed in the Permian, showed the first signs of breaking up in the Triassic, with the southern part, Gondwanaland, splitting off of Pangaea. But this was only the very early phase of the breakup, leaving most of the supercontinent intact.
The Triassic ended with yet another mass extinction not unlike that at the end of the Permian. Most Triassic dinosaurs died out as well as 75% of invertebrate species.
195,000,000 years ago: JURASSIC
Now come the dinosaurs most people are familiar with. As Pangaea split into Gondwanaland and Laurasia, gymnosperms still dominated the land. However, flowering plants (angiosperms) similar to magnolias, may have appeared by the end of the Jurassic. Modern groups of insects (flies, beetles, grasshoppers, termites, etc) now appear, perhaps coinciding with the rise of flowers. The first ancestors of the birds, including Archaeopteryx, appear. Birds evolved directly from the dinosaurs and can be considered the direct descendants of dinosaurs. It is now believed that dinosaurs really did not become extinct, they merely survived only in their feathered form.
Familiar dinosaur species that were present in the Jurassic were the carnivorous Allosaurus (a Saurischian, or "lizard-hipped" dinosaur of the Therapod group) and herbivorous Stegosaurus and Iguanodon (both Ornithischian, or "bird-hipped" dinosaurs),as well as Diplodocus and Apatosaurus (Saurischian).
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135,000,000 years ago: CRETACEOUS
Africa and India split from Gondwanaland and move towards Laurasia. North America drifts westwards, though its swampy interior was vastly different from the arid plains of today. The earth was warm and wet again, and sea levels rose.
Angiosperms flourished along with insects, which co-evolved with the flower. Oaks and maples joined the magnolia-like early angiosperms. Mammals continued much as they did in the Triassic and Jurassic—small, shrew-like. However, the split between placental and marsupial mammals occurs during the Cretaceous.
On the reptilian front, lizards and snakes appear. Cretaceous dinosaurs included the Ankylosaur and Triceratops (Ornithischians) and Tyrannosaurus (Saurischian Therapod).
The Cretaceous ended cataclysmically. Mass extinctions occurred yet again, including all of the (non-feathered) dinosaurs. Two lines of evidence suggest that this mass extinction was caused by a large meteor slamming into the earth. First, the Cretaceous-Tertiary (K-T) boundary that separates late Cretaceous rocks from rocks of the Tertiary era contains a very high level of the rare element Iridium. Iridium, rare on earth, is, however, found in meteorites. Second, an enormous impact crater has been found (filled up with earth and covered by jungle) in the Yucatan which dates near the K-T boundary. Such an impact would certainly be enough to cause such a mass extinction as is seen at the K-T boundary.
CENOZOIC ERA:
TERTIARY PERIOD:
65,000,000 years ago: PALEOCENE:
The dinosaurs ended at the K-T boundary, but the birds and mammals survived to fill the ecological niches left empty by the K-T mass extinction. Australia and Antarctica separate and begin their long isolation. The continents are beginning to take on an arrangement that we would find familiar.
Mammals begin to diverge into a wide range of groups, including marsupials, insectivores, and lemurs. Creodonts, a group ancestral to modern carnivores (dogs, cats, bears, etc.) appeared.
54,000,000 years ago: EOCENE
The Eocene was warm and wet with magnolias, figs and cycads common in warmer zones and redwoods , beech and elm common further North.
Many more mammalian groups familiar to us today evolved in the Eocene—horses, rhinos, camels, rodents, monkeys and whales. Whales evolved from land mammals of somewhat uncertain ancestry, though hippos might share a close common ancestor with the whales.
38,000,000 years ago: OLIGOCENE
The Great Tethys Sea, which had originally split Gondwanaland and Laurasia, has, by this time, shrunk to form the Mediterranean Sea. Elephants, old world monkeys and a species of ape first appear. Creodonts split into dog and cat lineages. All mammalian groups continue to evolve.
26,000,000: MIOCENE
Something happened between the Oligocene and the Miocene. The former was still a foreign, if increasingly familiar, world. With the Miocene we truly enter the modern (geologically speaking) world. Perhaps the difference is a cooling trend that allowed the Antarctic ice sheet to form. But, perhaps, the critical difference is the appearance of grasses.
Prior to the Miocene, there were forests, desert, tundra…all similar to what we see today. But there were no grasslands, savannas or steppes. Grasses evolved in the Miocene and the importance of this can be imagined if you remember that modern horses, goats, sheep and cattle all feed primarily on grasses and all modern grains are grasses. Thus the bulk of our diet derives from grasses. The appearance of grasses allowed the rise and proliferation of grazing animals, including the mastodon. Poisonous snakes also evolved at this time. All previous snake species had been constrictors.
12,000,000 years ago: PLIOCENE
The climate continued to cool and mammals continued to evolve. Grasslands and grazers spread.
QUATERNARY PERIOD:
2,500,00 years ago: PLEISTOCENE:
Includes the Great Ice Age when human ancestors (homonids) appear. Continents were about where they are today and mountains were about the same elevations as today. But glaciers covered large areas of the earth and the seas were low. Places like Yosemite Valley and the Great Lakes were formed by these glaciers. Species like mastodons, mammoths, saber-tooth tigers and ground sloths abounded, but were extinct by the end of the period. Those who visit Los Angeles should visit the La Brea Tar Pits and George Page Museum to learn more about this period. If I choose to do an article on human origins, I will go into more detail about this period.
Read About How We Know How Old Things Are (Dating Methods)
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