It is one thing to compare our bodies with those of frogs and fish. In a real sense we and they are much alike: we all have a backbone, two legs, two arms, a head, and so on. What if we compare ourselves with something utterly different, for example jellyfish and their relatives?
Most animals have body axes defined by their direction of movement or by where their mouth and anus lie relative to each other. Think about it: our mouth is on the opposite end of the body from our anus and, as in fish and insects, it is usually in the direction "forward."
How can we try to see ourselves in animals that have no nerve cord at all? How about no anus and no mouth? Creatures like jellyfish, corals, and sea anemones have a mouth, but no anus. The opening that serves as a mouth also serves to expel waste. While that odd arrangement may be convenient for jellyfish and their relatives, it gives biologists vertigo when they try to compare these creatures to anything else.
A number of colleagues, Mark Martindale and John Finnerty among them, have dived into this problem by studying the development of this group of animals. Sea anemones have been remarkably informative, because they are close relatives of jellyfish and they have a very primitive body pattern. Also, sea anemones have a very unusual shape, one that at first glance would seem to make them worthless as a form to compare to us. A sea anemone is shaped like a tree trunk with a long central stump and a bunch of tentacles at the end. This odd shape makes it particularly appealing, since it might have a front and a back, a top and a bottom Draw a line from the mouth to the base of the animal. Biologists have given that line a name: the oral-aboral axis. But naming it doesn't make it more than an arbitrary line. If it is real, then its development should resemble the development of one of our own body dimensions.
Martindale and his colleagues discovered that primitive versions of some of our major body plan genes—those that determine our head-to-anus axis—are indeed present in the sea anemone. And, more important, these genes are active along the oral-aboral axis. This in turn means that the oral-aboral axis of these primitive creatures is genetically equivalent to our head-to-anus axis.
One axis down, another to go. Do sea anemones have anything analogous to our belly-to-back axis? Sea anemones don't seem to have anything comparable. Despite this, Martindale and his colleagues took the bold step of searching in the sea anemone for the genes that specify our belly-to-back axis. They knew what our genes looked like, and this gave them a search image. They uncovered not one, but many different belly-to-back genes in the sea anemone. But although these genes were active along an axis in the sea anemone, that axis didn't seem to correlate with any pattern in how the adult animal's organs are put together.
Jellyfish relatives, such as sea anemones, have a front and a back as we do, a body plan set up by versions of the same genes.
Just what this hidden axis could be is not apparent from the outside of the animal. If we cut one in half, however, we find an important clue, another axis of symmetry. Called the directive axis, it seems to define two distinct sides of the creature, almost a left and a right. This obscure axis was known to anatomists back in the 1920s but remained a curiosity in the scientific literature. Martindale, Finnerty, and their team changed that.
All animals are the same but different. Like a cake recipe passed down from generation to generation—with enhancements to the cake in each—the recipe that builds our bodies has been passed down, and modified, for eons. We may not look much like sea anemones and jellyfish, but the recipe that builds us is a more intricate version of the one that builds them
Powerful evidence for a common genetic recipe for animal bodies is found when we swap genes between species. What happens when you swap a body-building gene from an animal that has a complex body plan like ours with one from a sea anemone? Recall the gene Noggin, which in frogs, mice, and humans is turned on in places that will develop into back structures. Inject extra amounts of frog Noggin into a frog egg, and the frog will grow extra back structures, sometimes even a second head. In sea anemone embryos, a version of Noggin is also turned on at one end of the directive axis. Now, the milliondollar experiment: take the product of Noggin from a sea anemone and inject it into a frog embryo. The result: a frog with extra back structures, almost the same result as if the frog were injected with its own Noggin.
Now, though, as we go back in time, we are left with what looks like a huge gap. Everything in this chapter had a body. How do we compare ourselves with things that have no bodies at all—with single-celled microbes?
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