For over a century, the conventional view has been that vertebrates are more closely related to echinoderms (starfishes, sea cucumbers, and sea urchins, for example) than to insects because both groups are “deuterostomes.” The other group, the protostomes, includes “insects, earth worms, molluscs and nematodes.” The basic difference has to do with embryology. In embryos, the mouth comes first in protostomes and second in deuterostomes. But now some researchers beg to differ:
While this textbook view of a close relationship between vertebrates and echinoderms has endured, some unexpected results from recent studies using comparisons of animal DNA to reconstruct evolutionary trees have questioned it. In parallel, some of the specific traits in how embryos develop that had been emphasised as being unique to the deuterostome branch of animals have been discovered in some species of protostomes.
These results suggest that the evidence that echinoderms are the closest relatives of vertebrates may perhaps be weaker than has long been believed …
To see whether the DNA data convincingly supports a close relationship between vertebrates and echinoderms, we looked at the number of changes in DNA that were found in both the vertebrates and the echinoderms but not in other animals. These shared characters, if found, would be evidence that would support the close relationship.
Looking at about 5,000 different genes, for approximately 70%, the protostome branch had more unique changes supporting it than the deuterostome branch did. That means the animals on the protostome branch share lots of unique changes to their DNA – and so this branch is very well supported by the DNA evidence. The close relationship between vertebrates and echinoderms, in contrast, is supported by much weaker evidence – they share relatively few unique DNA changes.
Max Telford, Paschalia Kapli, “Are we more closely related to starfish or insects? Our study questions 100 years of consensus” at The Conversation
They end up concluding,
What we conclude is that the confidence in a close relationship between vertebrates and echinoderms – in the textbooks for more than a century – is misplaced. We have shown that this evolutionary problem is particularly difficult to solve and that we vertebrates might turn out to be more closely related to snails and flies than we are to the starfish.
Max Telford, Paschalia Kapli, “Are we more closely related to starfish or insects? Our study questions 100 years of consensus” at The Conversation
If things are really uncertain at such a fundamental level (protostomes vs. deuterostomes), evolutionary biology could do with a lot less dogmatism in addressing the public.
The paper is open access.