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Scientists Have Found the First Branch on the Tree of Life

May 06, 2023May 06, 2023

Scientists have discovered which animal was the first to branch off from our collective common ancestor.

For years, debate had raged over whether the first to diverge was the sea sponge or the comb jelly.

Thanks to new chromosomal analysis techniques, we finally have an answer.

All animals on Earth share a common ancestor. Trace back the history of any creature from humans to slugs, and you’ll eventually be able to follow all of the branches on the animal tree of life back to its trunk.

But that trunk had to branch off at some point, or we wouldn't have all of today's animals. And that first split has been a bit elusive to scientists, due to it taking place around 600 million years ago.

We know a few things, though. Namely, we know that the first split resulted in the birth of two creatures—the ancestor of almost all animals, and the "sister" to that ancestor. That sister is the ancestor of just one group of modern-day animals.

For decades, scientists have debated which group of animals traces its lineage back to the "sister" of literally all other animals. They had two contenders for a long time—sea sponges and comb jellies. And now, thanks to new methods enabling researchers to analyze these creature's chromosomes, scientists believe they finally have the answer.

The key to this technique was looking not just at what genes each animal had, but where those genes were located on the creatures’ chromosomes. As a creature evolves, chromosomes will rearrange and genes will move around over time. But once they move, it's almost impossible for them to return to their original positions.

So, in essence, whichever animal showed the least re-shuffling of genes on chromosomes must have evolved into existence first. As we know we only have two options for that first split—sponges or comb jellies—whichever option has the least shuffling is the sister. First evolved equals first split.

In order to figure this out, 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. Those single-cell relatives would have been closely related to the "trunk" organism, and would preserve evidence of what that original genome would have looked like. The closer an organism is to that genome, the less it's changed, and the more likely it is to be the sister.

In both the non-animals and the comb jellies, researchers found 14 groups of genes located on separate chromosomes. But in the sponges, researchers found that those 14 groups had been rearranged into 7 groups, indicating that they split off from the original genome later than the comb jellies.

Which means that we have a winner! The sister to all other animals, the first to branch off, and the most genetically isolated animal is … drumroll please … the comb jelly!

Beyond serving as the answer to a long-agonized-over biological question, scientists are excited to investigate what this new knowledge can tell us about the history of animal evolution, and the mechanisms powering that evolution. The search for answers continues, but at least we now have one more in our pockets.

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