The Tree of Life’s First Branch
There was one commonancestor for all animals on Earth. If you trace the history of anycreature—from humans to slugs—you’d eventually follow all the branches of theanimal tree of life back to its trunk.
Coming from the otherdirection, that trunk had to branch off at some point, or we wouldn’t have adiverse collection of animals. Unfortunately, that first split has been elusiveto scientists because it took place around 600 million years ago.
Scientists have figuredout that the first split resulted in the birth of two creatures. One was theancestor of almost all animals, while its “sister” gave rise to just one groupof modern animals.
For decades, scientistshave debated which group of animals came from the “sister” ancestor. They hadtwo contenders—sea sponges and comb jellies. Thanks to new methods that enableresearchers to analyze these animals’ chromosomes, they think they have theanswer.
The key was to look notjust at what genes each animal had, but where those genes were located on thechromosomes. As a creature evolves, chromosomes will rearrange, and genes willmove around. But once genes move, it’s almost impossible for them to return totheir original position. Therefore, whichever animal showed the leastre-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 theplacements of certain groups of genes in sponges and comb jellies to theplacements of those same groups in their closest single-cell non-animalrelatives. (The single-cell relatives would have been closely related to the“trunk” organism and would have evidence of what that original genome lookedlike.) The closer an organism is to that genome, the less it has changed, andthe more likely it is the sister.
In both the non-animalsand the comb jellies, they found 14 groups of genes located on separatechromosomes. But in the sponges, they found those 14 groups were rearrangedinto 7 groups, which indicated they split from the original genome later thanthe comb jellies.
Consequently, thesister to all other animals, the first to branch off, and the most geneticallyisolated animal is the comb jelly.
Besides answering along-standing biological question, scientists can investigate what thisknowledge tells us about animal evolution, and the mechanisms of thatevolution. And so the search for more answers continues.
Thank goodness theyfigured that out. I don’t think I could have remained sane another day withoutknowing which animal came first.
