2.9: Fossil evidence for the history of life on earth

The earliest period in Earth’s history is known as the Hadean, after Hades, the Greek god of the dead. The Hadean is defined as the period from the origin of the Earth to the first appearance of life. Fossils provide our only direct evidence for when life first appeared. Such fossils are found in sedimentary rock; rock formed when fine particles of mud, sand, or dust entomb an organism. Hunters of fossils (paleontologists) do not search for fossils randomly; they use geological information to identify outcroppings of sedimentary rocks of the specific ages they are interested in.^[70]

Early in the history of geology, before Darwin and Wallace proposed their theory of evolution, geologists recognized that specific types of fossils were associated with rocks of specific ages. This correlation was so robust that rocks could be accurately dated based on the types of fossils they contained. Given the claims by theology-based young earth creationists that Earth is less than ~10,000 years old, it is worth reflecting on the interconnectedness of the sciences. Geologists do not rely solely on fossils to date rocks; many types of rocks do not contain fossils. Rocks can be dated based on the physics of isotope stability and the chemistry of atomic interactions. It uses the radioactive decay of elements with isotopes with long half-lives, such as ²³⁵Ur (uranium), which decays into ²⁰⁷Pb (lead), with a half-life of ~704 million years and ²³⁸Ur which decays into ²⁰⁶Pb with a half-life of ~4.47 billion years. Since these two Pb isotopes appear to be formed only through the decay of uranium, the ratios of uranium and lead isotopes can be used to estimate the age of a rock. In order to use isotope abundance to accurately date rocks, it is critical that all of the atoms in a mineral sample originated there and stayed there; none were washed into or out of the rock. Since uranium and lead have different chemical properties, this can be difficult to establish in some mineral types. That said, with care, and using rocks that contain chemically inert minerals, like zircons, the isotope ratio method can estimate the age of rocks to an accuracy of ~1% or better. Such age estimates support James Hutton’s (1726-1797) dictum that the Earth is ancient, with “no vestige of a beginning, no prospect of an end.”^[71] We know now, however, that this statement is not true; while very old, the Earth had a beginning, it coalesced around ~4.54 billion years ago, and it will disappear when the sun expands and engulfs it in about ~5.5 billion years from now.^[72]

Back to fossils. There are many types of fossils. Chemical fossils are molecules that, as far as we know, are naturally produced only through biological processes.^[73] Their presence in an ancient rock implies that organisms were present at the time the rock formed. Chemical fossils first appear in rocks that are between ~3.8 to ~3.5 x 10⁹ years old. What makes chemical fossils problematic is that there may be non-biological but currently undiscovered or unrecognized mechanisms that could produce them, so we need to be cautious in our conclusions.

Moving from the molecular to the physical, there are what are known as trace fossils. These can be subtle or obvious. Leaves and immotile organisms can settle on mud or sand and leave impressions. Burrowing and slithering animals make tunnels or disrupt surface layers. Walking animals can leave footprints in sand, mud, or ash. How does this occur? If the ground is covered, compressed, and converted to rock, these various impressions can become fossils. Later erosion can then reveal them. For example, if you live near Morrison, Colorado you can visit the rock outcrop known as Dinosaur Ridge and see fossil dinosaur footprints; there may be similar examples where you live!

We can learn a lot from trace fossils, they can reveal the general shape of an organism, its ability to move, or to move in a particular way. To move, an organism must have some kind of muscle or alternative mobility system and probably a nervous system that can integrate internal and external information and produce coordinated movements. Movement also suggests that the organisms that made the trace had something like a head and a tail. Tunneling organisms are likely to have had a mouth to ingest sediment, much like today’s earthworms - they were predators, eating the microbes they found in mud.

In addition to trace fossils, there are also the type of fossils that most people think about, which are known as structural fossils; the mineralized remains of the hard parts of organisms such as teeth, scales, shells, or bones. As organisms developed hard parts fossilization, particularly of organisms living in environments where they could be buried before being dismembered and destroyed by predators, scavengers, or microbes, became more likely.

Unfortunately for us (as scientists), many and perhaps most types of organisms leave no trace when they die. In part this may be because they live in places where fossilization is unlikely. Animals that live in woodlands, for example, rarely leave fossils. The absence of fossils for a particular type of organism does not imply that these types of organisms do not have a long history, but rather that the conditions where they lived and died was not conducive to fossilization. Many types of living organisms have no fossil record at all, even though, as we will see, there is molecular evidence that they arose tens to hundreds of millions of years ago.

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