The Fossil Record

A cast of the fossil remains of Archeopteryx, believed to be the link between dinosaurs and modern birds. It is considered to be one of the most important fossils ever discovered. (Courtesy: UC Berkeley)

The fossil record, despite its gaps, richly supports the concept of evolution. The earliest period of the existence of Life is forever out of reach: there are no rocks from that time with preserved information. Shortly after the end of the "heavy bombardment" period we find chemical fossils - carbon with a strange isotopic composition. Half a billion years later we find the first unicellular organisms: including their small reef-like mounds, the stromatolites. A billion years later we find clear signs of photosynthetic activity. The oxygen freed in the process stimulated the rise of eukarya, from collaborating bacterial precursors. By a billion years ago, eukarya are in full swing, and half a billion years later they have succeeded in generating pretty much all the familiar animals, plus many that are not familiar. Land plants start shortly after. Three-hundred million years ago we see the first reptiles. From there to birds and mammals is but a small step. Five million years ago the human ancestors split off from the ancestor of chimps, and since then we have learned to climb less, run more, and use our brain, which has since greatly increased in size.

Using the brain for modeling the solar system makes us unique among all the life forms. Although bees model the path of the Sun across the sky (to tell time and direction) they do so, presumably, without the faintest idea of the nature of the Sun.

Mosquito in amber, fossilized tree resin from the Cretaceous Period approximately 92 million years old. (Courtesy: S. Kurth)

Among the most intriguing objects on our planet are rocks shaped like organisms. They are called "fossils" which means "things dug up" in Latin. For example, from the Jurassic deposits of southern Germany, given time and luck, one can collect an entire stony zoo of what looks like marine animals. Some of these are readily recognized as resembling sponges, corals, bivalves (mussels and clams and such), snails, sea urchins, brittle stars, lamp shells and other types of creatures commonly found along the sea shore. Others, more rarely found, look like parts of fishes or reptiles (teeth, vertebrae). Very lucky collectors have found entire fish, diving reptiles (ichthyosaurs) and horseshoe crabs, even something that looks like a cross between a bird and a reptile.

But the most common type of fossil, from the bottom to the top of the sequence, are coin- to fist-sized spiral-shaped objects resembling a tightly wound ram's horn. The locals, for centuries, called these objects "shtah shnegli" which means "stone snails". Geologists call them "ammonites", after the ram's horns of the Egyptian god Amun, spelled Ammon by the Greeks. (As an aside: the word ammonia and its derivatives has the same origin: ammonium salts were originally obtained from horns of sheep and deer.)

Ammonite fossil from the late Jurassic Period, approximately 200 million years old. (Courtesy: British Geological Survey)