Comb Jellies are Likely the Oldest Living Relative of the First Animal Ever

If true, this not only settles a long held scientific debate, but could change the way we think about how evolution works.

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One of our many eternal quests as human beings is to understand Life. What makes something “alive?” What causes groups of cells to become conscious, and how do we define it? What was the very first animal that all animals evolved from? Experts don’t yet have answers to these questions, but they’re getting closer than ever before to finding out.

New research might finally settle the ongoing debate between experts regarding which animal was the first to ever evolve. Over the years, scientists have narrowed the field to three possibilities. Still, conflicting research has made it challenging to determine which of the three animals was the very first to evolve — until now.

The Debate

Fossil records indicate that the first animals evolved sometime around 600 million and over 700 million years ago, and many scientists agree that the closest living relatives of the first animals to evolve on Earth were sea anemones, sponges, and comb jellies (ctenophores). But that was about as far as we got.

Which of these three animals is truly the first has stumped evolutionary biologists for years. Finding answers isn’t easy either because squishy animals such as all three of the prospects don’t exactly leave great fossil records due to the lack of bones.

The absence of evidence leaves scientists wondering which animals were the first to branch off from the One common ancestor to begin evolving in a new direction. Researchers have found ways to search for answers regardless of the limited fossil records and have further narrowed the possibilities down to either sponges or comb jellies — side note: despite their name, comb jellies are not closely related to jellyfish.

The problem now is that competing studies conflict in their conclusions. Some support sponges, while others support comb jellies. An evolutionary biologist at Yale University, Casey Dunn, explains:

“Understanding these deepest relationships in the animal tree of life is absolutely critical for reconstructing the history of the origin and evolution of a lot of the complex traits that we’re most interested in — things like the nervous system and animal symmetry.”

According to Dunn, over the last century, the implied assumption was that complex features within an animal’s lineage mostly evolved in straightforward steps. Because of this assumption, experts believed sponges were the more likely option to be the earliest animal compared to comb jellies.

Unlike comb jellies, sponges lack muscles, neurons, and a nervous center, but they do have cells called choanocytes, which are similar to single-celled non-animals called choanoflagellates.

Sponges’ simple structure often leads to them being mistaken for plants. This contributes to the idea that they must have diverged from the first animal’s lineage before muscles, nervous systems, and neurons evolved. Meanwhile, comb jellies, which possess all of the above, branched off sometimes later.

Still, some research suggests that despite these seemingly rational assumptions, it was the comb jelly that branched off first.