Scientists Debate the Origin of Cell Types in the First Animals

Scientists Debate the Origin of Cell Types in the First Animals

Klaus Stiefel

For well over a century, it has been widely assumed that the ancestors from which the first animal evolved were simple blobs of identical cells. Only later, after the animals formed their own branch on the tree of life, did those cells start to differentiate into various cell types with specialized functions. But now, painstaking genomic analyses and comparisons between the most ancient animals alive today and their closest non-animal relatives are starting to overturn that theory.

The recent work paints a picture of ancestral single-celled organisms that were already amazingly complex. They possessed the plasticity and versatility to slip back and forth between several states — to differentiate as today’s stem cells do and then dedifferentiate back to a less specialized form. The research implies that mechanisms of cellular differentiation predated the gradual rise of multicellular animals.

Now, scientists are reporting the most compelling evidence yet for the new narrative. Their work, and the debate inspired by its publication in Nature last month, also highlights how difficult it is to pin down definitive answers to these kinds of evolutionary questions — and how wide a net researchers have to cast in pursuit of those answers.