Key cellular machinery predated rise of animals

With the help of an obscure microorganism with ancient roots, scientists have discovered that critical biological processes at work today in humans and other animals were in place before the advent of multicellular life on Earth hundreds of millions of years ago.

The findings, published today (July 18) in the journal Science, provide a powerful new line of evidence that many of the genetic and cellular processes that exist in animals today arose before their origin from a common unicellular ancestor.

“We’ve found that the cell biology tool kit was pretty sophisticated before the dawn of animals,” says Sean Carroll, the senior author of the Science report and a professor of genetics at UW–Madison’s Howard Hughes Medical Institute. “There is no doubt these molecules have been co-opted. The same pathways we see in animals today were in place and working 600 million years ago.”

Plumbing the molecular workings of a modern, single-celled organism known as a choanoflagellate, Carroll and co-authors Nicole King and Christopher T. Hittinger found the same key signaling pathways that enable cells in animals to communicate and interact with one another. Such pathways, found previously only in animals, are involved in everything from cancer to the development of sensory functions such as hearing.

“These were thought to be animal inventions,” Carroll says. “But it seems that these pathways were in place and working” long before multicellular animals with distinct body plans and systems of organs arose from an ancient soup of microscopic protozoan life.

While the use of these signaling pathways are generally known in animals, and are much studied by scientists, their functions in choanoflagellates remain a mystery, says King, the lead author of the Science report.

Choanoflagellates are a phylum of transparent, single-celled microbes that propel themselves with whip-like appendages. A closer relative to animals than other single-celled organisms, they have an ancient lineage whose common ancestry with all animals dates back at least 600 million years.

The new study chips away at a central question in animal evolution: how multicellular animals evolved from a protozoan ancestor.

One notion of how that occurred, according to King, is that molecular pathways important to multicellular development evolved first in unicellular ancestors functioning, perhaps, in support of such things as food acquisition, mating and cell communication. Later, these pathways were co-opted for new roles in animal development.

“This is consistent with the idea of evolution as a tinkerer, cobbling together tools that are already available, rather than inventing a new widget for each job,” King explains.

“The expression in choanoflagellates of proteins involved in cell interactions in (animals) demonstrates that they evolved before the origin of animals and were later co-opted for development,” the group writes in the Science report.

Using the tools of modern molecular biology, scientists are developing new ways to look back in time, to ages that predate physical evidence such as fossils. These powerful techniques are beginning to help flesh out the details of life during a time from which there is virtually no fossil record.

For instance, the use of choanoflagellates, says Carroll, is an exercise in comparative genomics, which can help identify “the minimal set of genes in place at the outset of evolution by revealing those shared by all animals and their nearest relatives.”