I’ve been taking an OLLI (Osher Life Long Learning institute) class this month, titled Major Evolutionary Transitions. The presenters are two of my favorites: Dr. Sarah Fowell and Dr. Patrick Druckenmiller, who gave a wonderful course on dinosaurs last year. This time we’re skipping the dinosaurs and looking at eight topics, two a week. The first two were the Ediacaran fauna, soft-bodied animals that lived before animals evolved hard parts that are fossilized in conventional ways, and the Cambrian explosion, the first animals with hard parts.

To look at this transition, we have to look at fossils as more than fossilized bone or shells. That kind of fossil does indeed start with the Cambrian, about 542 million years ago. But there are other kinds of fossils.


Chemical fossils are chemicals produced by life processes, and those go back over 3 billion years. Traces resembling microbes can be found in rocks dated to 3.5 billion years of age. Stromatolites — limestone structures similar to those built today by cyanobacteria — date back to 3.5 billion years ago. (Cyanobacteria? Ever had a fish tank? Think blue-green algae, though they’re not really algae.)

By the late Precambrian multi-celled animals were common, but they had no hard body parts. It takes really exceptional conditions to fossilize a jellyfish or a sponge, but rare fossils of these animals can be found. Then, 542 million years ago, there was a rapid increase first is the number of trace fossils—such as burrows or tracks—and then in actual hard parts. What caused this?

Most likely, a change in sensory input. The senses of smell/taste, being chemically based, are probably very old. I have imagined a planet in Homecoming, Mirror, at a very early stage of evolution where the only hard parts are those secreted by the stromatolite-like land corals. At least one of the soft-bodied animals will swim up-gradient to a particular chemical and attempt to ingest the source of the odor. Chances are that some of the Precambrian animals had similar abilities, and enough nervous system to use them.

Light sensing cells might also have been useful, in avoiding excessive light or in seeking light. (Moving up or down in the water column.) But the evolution of lenses and the compound eyes they made possible may well have set off the evolutionary arms race.

Precambrian animals, as far as we know, did not hunt each other. They may well have eaten each other, as anemones do today. But how could they find each other? And what is the use of armor or skeletons if there is nothing trying to eat you and you don’t need to move rapidly?

Then something evolved the first primitive eyes. It probably wasn’t the best-known predator of the Cambrian, the Anomalocaris, though it may well have been one of its ancestors. Once the eyes were there, even in very primitive form, the hunter could find its prey. The prey animals, such as trilobites, had to evolve eyes as well, to see the predators coming, and hard coverings, to make them harder to eat when caught. The prey animals got bigger to avoid being eaten, and the predators got bigger to catch them. The resultant arms race both speeded up the pace of evolution and produced increasingly large hard parts to be fossilized.

It’s worth pointing out that it took a long time for the Anomalocaris to be recognized as a single animal. Initially, the “arms” near the mouth were thought to be a shrimp-like creature, the ring of plates that made up the mouth were seen as a sort of jellyfish, and the body was thought to be a kind of sea cucumber. The animal was not put together until the 1980’s.

The photos are screenshots from an aquarium program, Aquazone Blue Planet which, in addition to conventional aquariums, has several Cambrian animals and backgrounds.