What is so fascinating about family trees
Will the animal pedigree be rewritten?
Revolutionary discovery: Contrary to previous assumptions, the comb jellyfish are apparently the sister group of all other animals. They were the first to split off from the animal family tree in evolution - and not the sponges, as previously thought. If this result of a DNA study is confirmed, not only would the family tree of life have to be rewritten - the evolution of nerves and muscles would then also proceed differently, as the researchers report.
Rib jellyfish are as fascinating as they are mysterious: the semitransparent galert bodies of these marine animals, with their iridescent ribs, appear graceful and harmless - and with their pulsating lights they somewhat resemble a delicate alien spaceship. But the ctenophores are rabid predators: In the Baltic Sea, the immigration of the comb jelly Mnemiopsis Leidyi could even endanger the cod population. The approximately 200 known species of the comb jellyfish are also very adaptable and have colonized almost all marine ecosystems.
Very old and yet progressive
But how do the comb jellyfish fit into the family tree of animals? Biologists have been puzzling and arguing about this question since the discovery of this peculiar group of animals. On the one hand, fossil finds suggest that ctenophores belong to the oldest animal groups on earth. The first ancestors of the comb jellyfish lived around 550 million years ago - and thus in the time when the first multicellular cells emerged.
On the other hand, comb jellyfish have muscles and a nervous system and thus seem more developed than, for example, the sponges. For a long time, the sponges were therefore considered to be the most primitive large group in the animal kingdom and the ctenophores were assigned to a league with more developed forms such as cnidarians and the bilateria - the large group to which we also belong.
Genetic pedigree reconstructed
Now, however, a new genetic analysis is overturning this largely established family tree. Nathan Whelan of US Fish and Wildlife Service and his colleagues sequenced the genome and transcriptome of 27 comb jellyfish and compared them to the genomes of ten other species of comb jellyfish and 50 species from the other four major groups in the animal kingdom. With the help of these sequence comparisons, they reconstructed when which large group split off in the family tree.
The surprising result: It was not the supposedly primitive sponges that were the first to separate from the test of the animal world, but the rib jellyfish. "The ctenophores are thus the sister group of all other animal groups that exist today on earth," state Whelan and his colleagues. That could mean that the family tree of the animals has to be rewritten.
Nervous system arisen twice?
But this result not only has consequences for the family tree of life. It also sheds new light on the evolution of the nervous system and muscles. Because if the comb jellyfish are less closely related to all higher animals than the sponges and placozoa, then the nervous system cannot have developed just once, as previously assumed.
Instead, the comb jellyfish and the common ancestors of cnidarians and bilateria must have developed these biological structures convergent - and thus independently of one another. How and why this happened and why the nerves and muscles of these family tree branches are nevertheless relatively similar must now be shown by further research.
“Placing the comb jellyfish as sister group to all other animals erodes the classic but deeply flawed notion that we can easily move animals from supposedly old, 'lower' organisms to 'higher' and more developed," comments Casey Dunn of Yale University . (Nature Ecology & Evolution, 2017; doi: 10.1038 / s41559-017-0331-3)
(Nature, 10/11/2017 - NPO)October 11, 2017
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