Earth's earliest animals resembled the marine creatures known as comb jellies, according to a new study, rather than sponges as the dominant side of a long debate has held.
Like the mystery of how life began, the nature of the first animals is one of the great questions of biological history. Two major theories on this have emerged: One holds we are descended from sponges, the other that our oldest animal ancestors were more like comb jellies (ctenophores not to be confused with cnidarians or jellyfish). Alternatives have also been proposed.
It's a difficult question to answer, because neither sort of invertebrate fossilizes well and the passage of time has destroyed most of the records that did form. Nevertheless, most scientists working on the problem favor the sea sponge theory, based on evidence from various sources.
However, a paper in Nature Ecology and Evolution has shown the debate is not over. Senior author Dr Antonis Rokas of Vanderbilt University reached the conclusion that ctenophores lie at the base of the animal family tree, using a new method he worked out to compare the DNA of species from different branches.
"The current method that scientists use in phylogenomic studies is to collect large amounts of genetic data, analyze the data, build a set of relationships and then argue that their conclusions are correct because of various improvements they have made in their analysis," Rokas said in a statement. In most cases, this has produced consensus on these relationships, but there are exceptions where separate attempts have led to seemingly irreconcilable differences.
Rokas considered seven examples where geneticists had been unable to agree on the relationship between animal species, along with five from plants and six from fungi, and examined individual genes that were seen as indicating family ties.
The problem, Rokas concluded, was that in some cases there are a small number of genes that strongly indicate one relationship, while a far larger number are consistent with something different, but less convincingly so. These few “opinionated genes” are preventing us from resolving some questions, and may be indicative of extremely rapid diversification, making some of the contentious debates almost impossible to settle.
Other relationships, however, proved easier to unravel. From hundreds of thousands of genes examined in this way, Rokas and his co-authors concluded there are substantially more that align with the jellies as ancestors theory than the sponge hypothesis.
Using the same technique, Rokas found 74 percent of crocodile genes indicate a closer relationship to turtles than to birds. This conclusion comes as no surprise to anyone brought up on the idea of reptiles as a distinct division of vertebrates, but has been greatly debated among geneticists in recent times.