Now that we have put all the strata in order and estimated their dates, we can read the fossil record from bottom to top. Figure 3 shows a simplified timeline of life's history, depicting the major biological and geological events that occurred since the first organisms arose around 3.5 billion years ago.6 This record gives an unambiguous picture of change, starting with the simple and proceeding to the more complex. Although the figure shows the "first appearances" of groups like reptiles and mammals, this shouldn't be taken to mean that modern forms appear in the fossil record suddenly, arising out of nowhere. Instead, for most groups we see gradual evolution from earlier forms (birds and mammals, for example, evolved over millions of years from reptilian ancestors). The existence of gradual transitions between major groups, which I discuss below, means that assigning a date to a "first appearance" becomes somewhat arbitrary.
The first organisms, simple photosynthetic bacteria, appear in sediments about 3.5 billion years old, only about a billion years after the planet was formed. These single cells were all that occupied the Earth for the next two billion years, after which we see the first simple "eukary-otes": organisms having true cells with nuclei and chromosomes. Then, around 600 million years ago, a whole gamut of relatively simple but multicelled organisms arise, including worms, jellyfish, and sponges. These groups diversify over the next several million years, with terrestrial plants and tetrapods (four-legged animals, the earliest of which were lobe-finned fish) appearing about 400 million years ago. Earlier groups, of course often persisted: photosynthetic bacteria, sponges, and worms appear in the early fossil record, and are still with us.
figure 3. The fossil record showing first appearance of various forms of life that arose since the Earth formed 4,600 million years ago (MYA). Note that multicellular life originated and diversified only in the last 15% of life's history. Groups appear on the scene in an orderly evolutionary fashion, with many arising after known fossil transitions from ancestors. The sequence shown, along with the transitional forms, disproves creationist claims that all forms of life arose not only suddenly, but also at the same time.
Fifty million years later we find the first true amphibians, and after another fifty million years reptiles come along. The first mammals show up around 250 million years ago (arising, as predicted, from reptilian ancestors), and the first birds, also descended from reptiles, show up fifty million years later. After the earliest mammals appear, they, along with insects and land plants, become ever more diverse, and as we approach the shallowest rocks, the fossils increasingly come to resemble living species. Humans are newcomers on the scene—our lineage branches off from that of other primates only about seven million years ago, the merest sliver of evolutionary time. Various imaginative analogies have been used to make this point, and it is worth making again. If the entire course of evolution were compressed into a single year, the earliest bacteria would appear at the end of March, but we wouldn't see the first human ancestors until 6 a.m. on December 31. The golden age of Greece, about 500 bc, would occur just thirty seconds before midnight.
Although the fossil record of plants is sparser—they lack easily fossilized hard parts—they show a similar evolutionary pattern. The oldest are mosses and algae, followed by the appearance of ferns, then conifers, then deciduous trees, and, finally, flowering plants.
So the appearance of species through time, as seen in fossils, is far from random. Simple organisms evolved before complex ones, predicted ancestors before descendants. The most recent fossils are those most similar to living species. And we have transitional fossils connecting many major groups. No theory of special creation, or any theory other than evolution, can explain these patterns.
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