What triggered the Cambrian Explosion?

Almost all of today's animal body types evolved in a single period less than ten million years long. This "explosion" of diversity occurred with seemingly random timing in a period tens of millions of years long. The  abruptness of the Cambrian Explosion was a severe puzzle to Darwin, and has remained troubling ever since.

Why did the Cambrian Explosion happen when it did? If it was so abrupt--just waiting to happen--why didn't it happen earlier? Chris Phoenix thinks he knows the answer.

585 million years ago (MYA), the earth was heavily glaciated, and life (as far as we know) had not progressed past the single-celled stage. By 575 MYA, the first multicellular fossils have been found. But the Cambrian Explosion waited until about 545 MYA.

There were, of course, several enabling factors, such as the retreat of glaciation, the increasing oxygen content of the atmosphere, and the gradual complexification of organisms. But all of these were slow processes, or else predated the Cambrian Explosion by tens of millions of years.

It seems likely that there was a trigger intrinsic to organisms--some capability that developed in some lineage, that took time for evolution to discover, but that was highly advantageous once it arrived--advantageous enough to allow that lineage to diversify wildly, and to outcompete many other lineages.

Cellular differentiation--the ability of a cell to lock itself into a particular specialized type, without requiring ongoing chemical signaling--is proposed as a suitable trigger for the Cambrian Explosion.

Jellyfish and hydras, which predate the Explosion, have cells that are specialized but not differentiated. Differentiated cells would allow intricate multi-scale structures--organs and networks--to be maintained with far less energy cost. However, differentiation would be a losing proposition for single cells, so would have taken a while to evolve.

The differentiation hypothesis also has implications for embryology and stem cell research.

Chris will also be giving several other talks at this BIL conference.