The Tree of Life’s First Branch

There was one common ancestor for all animals on Earth. If you trace the history of any creature—from humans to slugs—you’d eventually follow all the branches of the animal tree of life back to its trunk.

Coming from the other direction, that trunk had to branch off at some point, or we wouldn’t have a diverse collection of animals. Unfortunately, that first split has been elusive to scientists because it took place around 600 million years ago.

Scientists have figured out that the first split resulted in the birth of two creatures. One was the ancestor of almost all animals, while its “sister” gave rise to just one group of modern animals.

For decades, scientists have debated which group of animals came from the “sister” ancestor. They had two contenders—sea sponges and comb jellies. Thanks to new methods that enable researchers to analyze these animals’ chromosomes, they think they have the answer.

The key was to look not just at what genes each animal had, but where those genes were located on the chromosomes. As a creature evolves, chromosomes will rearrange, and genes will move around. But once genes move, it’s almost impossible for them to return to their original position. Therefore, whichever animal showed the least re-shuffling of genes on its chromosomes must have come into existence first. Out of the two options, whichever animal has the least shuffling is the sister.

The team compared the placements of certain groups of genes in sponges and comb jellies to the placements of those same groups in their closest single-cell non-animal relatives. (The single-cell relatives would have been closely related to the “trunk” organism and would have evidence of what that original genome looked like.) The closer an organism is to that genome, the less it has changed, and the more likely it is the sister.

In both the non-animals and the comb jellies, they found 14 groups of genes located on separate chromosomes. But in the sponges, they found those 14 groups were rearranged into 7 groups, which indicated they split from the original genome later than the comb jellies.

Consequently, the sister to all other animals, the first to branch off, and the most genetically isolated animal is the comb jelly.

Besides answering a long-standing biological question, scientists can investigate what this knowledge tells us about animal evolution, and the mechanisms of that evolution. And so the search for more answers continues.

Thank goodness they figured that out. I don’t think I could have remained sane another day without knowing which animal came first.

 

https://www.msn.com/en-us/news/technology/scientists-have-found-the-first-branch-on-the-tree-of-life/ar-AA1LAkS2?ocid=hpmsn&cvid=cddbc1e105374c51afa382b8cdd3674d&ei=18