What came first, the sponge or the comb jelly?
4 May 2017 by Evoluted New Media
Scientists have reignited a zoology debate by claiming the earliest branch of the animal family tree was not sponges but instead marine invertebrates called comb jellies.
Scientists have reignited a zoology debate by claiming the earliest branch of the animal family tree was not sponges but instead marine invertebrates called comb jellies.
This claim was made after analysing thousands of genes belonging to sponges and comb jellies, a method known as phylogenomics. The researchers discovered that comb jellies have more genes that support their ‘first to diverge’ status than sponges.
Professor Antonis Rokas, from Vanderbilt University and author of the paper, said: “The current method that scientists use in phylogenomic studies is to collect large amounts of genetic data, analyse 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. This has worked extremely well in 95% of cases, but it has led to apparently irreconcilable differences in the remaining 5%.”
In the past, scientists organised the animal family tree based on their relative complexity, and because of their relative simplicity, sponges were believed to be the earliest members. As a result of large amounts of genomic data becoming more widely available, a new technique – phylogenomics – developed. This technique enabled many evolutionary relationships to be clarified but has also led to ongoing debates about the status of certain organisms on the tree of life.
Since then, arguments have been made declaring both to be the earliest animal species. Professor Rokas said: “In these analyses, we only use genes that are shared across all organisms. The trick is to examine the gene sequences from different organisms to figure out who they identify as their closest relatives. When you look at a particular gene in an organism, let's call it A, we ask if it is most closely related to its counterpart in organism B? Or to its counterpart in organism C? And by how much?"
The researchers determined what support each gene provided to another hypothesis, comb jellies first, over another, sponges first. The resulting difference was called a phylogenetic signal with a correct hypothesis one most consistently favoured by the phylogenetic signals. As part of this research, the scientists found that 74% of the shared genes between crocodiles and birds show they are sister lineages, while turtles are close cousins. The research was published in Nature Ecology and Evolution.
