But the acorn worm still managed to surprise scientists.
It's -- well -- a little closer on our family tree than one might expect.
A study published Wednesday in Nature reports that the common ancestor of today's slimy marine worms shared some 70 percent of our genes. That's kind of a lot: Chimps share 98 percent of our genome, but roundworms share just 21 percent. We share more genes with ancient marine worms than we share with chickens. I bet you never thought you'd be so disappointed to see a chicken get bumped lower on your list of cousins.
You don't have to worry about the acorn worm showing up for Thanksgiving this year. The animals live in burrows on the ocean floor. And thank goodness, because their table manners are atrocious: They swallow sand full of microorganisms and decaying organic matter to get their nutrients, and then let out long "casings," also known as poop, into the ocean -- or out of it, if you catch one at low tide.
Like us, acorn worms are deuterostomes. But the last common deuterostome ancestor lived about 500 million years ago, and we've been diverging from our cousins ever since. The so-called acorn worms are an old group -- probably about as old as the last common ancestor, or at least pretty close -- so they provide a nice window into our wormy genetic heritage. But the real question is how so much could change on the outside while so many genes stayed exactly the same.
Make no mistake: A 30 percent different still leaves room for staggering changes. After all, we needed far fewer unique genes to turn out quite different from our closest cousins in the animal kingdom.
But by studying the gene clusters that have been preserved over time -- even as we evolved into much more complex critters -- scientists can try to figure out what wormy adaptations our nifty human traits emerged from.