Idle Musings on Evolutionary Time

Since Aristotle, students of nature have been tempted to rank some organisms as somehow “better” than others. Aristotle ranked all organisms from most simple to most complex. In Medieval Europe, his ideas were taken up and incorporated into a grand scala naturae or ladder of life, with lowly worms at the bottom, humans at the top of the mortal beings, and angels above us.

Evolutionary theory has had its share of attempts to understand the scala naturae, usually with time playing the role of the force that makes some organisms more “evolved” than others. Lamarck posited multiple origins of life over the ages and suggested that the lowliest species are newcomers on the world’s stage, whereas loftier species had been around for longer and attained greater heights. Hints of this view still resonate in popular misconceptions about evolution.

Of Frog Eyes and Jellyfish Tentacles

Author’s note: There were errors in the original version of this article. Please see the Post Scripts for more details.

---

In 1901, Hans Spemann revolutionized biology by doing something very strange. He had been watching various embryos grow, and he got bored. That’s the short version of his motivation.

Basically, the embryologists of the day had already spent oodles of time carefully documenting the normal development of various animals from egg to embryo to hatchling. They had established that most animals – vertebrates included – go through a succession of embryological stages called the morula, blastula and gastrula. The morula is just a dense ball of cells (In Latin, morula means “mullberry”), the blastula is a hollow ball of cells, and the gastrula is like a blastula with an indentation somewhere. The indentation keeps growing inward until it meets the other side and becomes a tube that runs through the embryo’s whole body. This tube becomes the digestive tract, which, if you think about it, is just a tube running through an animal’s whole body. In some species, called the protostomes, the original indentation becomes the mouth, but in other animals (deuterostomes) it becomes the anus. We are deuterostomes. In fact, it’s all very interesting, because the same program seems to occur in wildly different organisms, from worms to molluscs to starfish and humans.