In the beginning, life was simple. Then, about half a billion years ago, biology got complicated. Identical cells shrugged off the shackles of conformity and divided the chores between them, evolving into the first members of the animal kingdom
Exactly what this animal prototype looks like has long been the subject of debate. A new study has now revealed the line between ancient microbes and primordial animals might have been crossed by a early form of stem cell
Today, biologists tend to support one of two contenders for 'Earth's First Animal'. In one corner, there's Team Sponge. In the opposing corner, Team Comb Jelly
Each camp has good reasons to suspect the ancestor of kingdom Animalia has resembled their particular phylum more closely than any other
Sponges have the kind of anatomy we would imagine of a primitive animal. Comb the jellies look far more complex, but have remarkably ancient-looking genomes.
Biologists from the University of Queensland in Australia have now looked beyond the genetic code within sponge cells by analyzing what is known as a transcriptome ̵
"This technology has been used only for the last few years, but it's helped us finally address an age-old question, discovering something completely contrary to what anyone had ever suggested, "says biologist Sandie Degnan
"Now we have the opportunity to re-imagine the steps that gave rise to the first animals, the underlying rules that turned the single cells into multicellular animal life."
As with all animals, the cells making up our own bodies are individually suited to performing specific tasks. (19659003) For more than a century, biologists imagined those first differentiated colonies as a close relative of modern members of the phylum Porifera – the humble sponge
Since the cells called choanocytes lining their guts look suspiciously like free-living, tail-whipping organisms called choanoflagellates, biologists figured the case was closed on "the world's first animal."
"Biologists for decades believed the current theory was a no-brainer, as sponge choanocytes look so much like single-celled choanoflagellates – the organism considered to be the closest living family of the animals, "says Degnan
But as biologists all know, looks can often be decelliging
Choanoflagellates might look like they just teamed up one sunny day 600 million years ago to make a sponge, but research has shown there are signi ficant differences in their respective genomes and biochemistry
Amphimedon queenslandica sponge tissue with those of a choanoflagellate and two other similar, single celled organisms
This was less like comparing the genetic libraries of sponge and pre-animal cells, and more like analyzing their library cards to see if they were reading from a shared book list. And it turns sponge gut cells and choanoflagellates are not in the same book club after all
Even more surprising was the library card belonging to an undifferentiated type of sponge cell called a pluripotent mesenchymal archaeocyte.
"We found that the first multicellular animals were probably not like modern-day sponge cells , but were more like a collection of convertible cells, "says Degnan's colleague, marine scientist Bernie Degnan.
" The great-great-great-grandmother of all the cells in the animal kingdom was probably quite similar to and the stem cell. "
In some respects, this makes a lot of sense. Animals are made of a larger variety of cell types when compared to plants and fungi.
Developing a talent for swapping faces could have given our animal ancestors an edge as they grouped. It is possible that the first animals set themselves apart by evolving regulatory systems that allowed multiple forms of differentiated cells to exist at the same time in the same population.
Understanding this has implications beyond our evolution.
It is early days for Team Stem Cell, but it is clear that the debate over the earliest animal is far from settled
This study was published in Nature