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At first, life was simple. Then, about a billion years ago, biology is complicated. Identical cells abandoned the chains of conformity and shared the household tasks, thus becoming the first members of the animal kingdom.
The exact nature of this animal prototype has long been debated. A new study has now revealed that the boundary between ancient microbes and primordial animals could be crossed by an early form of stem cell.
Today, biologists tend to support one of two contenders for "the first animal on the Earth". In one corner, there is Team Sponge. In the opposite corner, Team Comb Jelly.
Each camp has good reason to suspect that the ancestor of the kingdom, Animalia, is more like any other than their phylum.
The sponges have the kind of anatomy that we could imagine from a primitive animal. Comb jellies seem much more complex, but have remarkably old genomes.
Biologists from the University of Queensland, Australia, have gone beyond the genetic code in sponge cells by analyzing what's called a transcriptome – a description of the genetic activity when cells live their lives. life.
Transcriptome analysis is becoming the tool of choice for researchers who want to understand the unique behavior of identical genome cells.
"This technology has only been used for a few years, but it finally helped us to solve a secular question, to discover something totally contrary to what anyone had proposed," says biologist Sandie Degnan.
"We now have the opportunity to re-imagine the steps that gave rise to the first animals, the underlying rules that turned individual cells into a multicellular animal life."
As with all animals, the cells composing our own body are adapted to specific tasks. This differentiation distinguishes our organisms from simple, uniform agglomerates of cells that agglutinate in the hope of digital security.
For more than a century, biologists have imagined these first differentiated colonies as a close relative of the modern members of the phylum Porifera – the humble sponge.
Since the cells called choanocytes that line their intestines look strangely like autonomous organisms calling for choanoflagellates, whips at the tail, biologists have assumed that the case was closed on the "first animal of the world".
"For decades, biologists have felt that the existing theory was a no-brainer, as spongy choanocytes look so much like unicellular choanoflagellates – the organism considered to be the closest living relative of animals," says Degnan.
But as all biologists know, appearances can often be misleading.
Choanoflagellates may seem to have simply teamed 600 million years ago on a sunny day to make a sponge, but research has shown that there are significant differences in their respective genomes and biochemistry.
To further test the connection, Degnan and his research team compared transcriptomes, behaviors, and life cycles of three different types of Amphimedon queenslandica terry cloth with those of a choanoflagellate and two other similar, unicellular organisms.
It looked less like a comparison of the genetic libraries of sponge and pre-animal cells, but rather an analysis of their library cards to determine if they were reading from a shared book list. And that turns the cells of the intestinal sponge and choanoflagellates are not in the same reading club after all.
Even more surprising was the library card belonging to an undifferentiated type of sponge cells called pluripotent mesenchymal archeocyte. The indecisive cells of the sponge looked much more like an ancestor of the first animals than those transformed into intestinal or cutaneous cells.
"We found that the first multicellular animals probably did not look like modern sponge cells, but rather a collection of convertible cells," says Degnan's marine scientist Bernie Degnan.
"The great-great-great grandmother of all the cells of the animal kingdom, so to speak, was probably quite similar to a stem cell."
In some ways, it makes a lot of sense. Animals are made up of a greater variety of cell types than plants and fungi.
Developing a talent to quickly exchange faces could have given an advantage to our animal ancestors in their grouping. It is possible that the first animals are distinguished by evolving regulatory systems that allow several differentiated forms of cells to exist at the same time in the same population.
Understanding this has implications beyond our evolution. It could also help us understand the complexities of cancer.
Stem cells are just beginning, but it is already clear that the debate over the oldest animal is far from settled.
This research was published in Nature.
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