What’s in a head? According to new research, a bit of the tail of our ancestors.
In the early days of complex, multicellular life on Earth, animals started out without spines or brains. They only had one network of neurons distributed throughout their bodies. Over millions of years, however, this system has somehow become concentrated at one end. But how?
Tunicates, or “sea squirts,” are the closest living relatives of vertebrates, and they don’t have true heads.
Rather, their central nervous system is made up of clusters of neurons in the anterior and posterior parts of their body, with a dorsal strand connecting them both. In adulthood, these animals look like stagnant sponge-like spots without a clear head or tail. But as tadpole-like larvae, their brains are easier to distinguish.
“Tunicates are like an evolutionary prototype for vertebrates,” says zoologist Ute Rothbächer from the University of Innsbruck in Austria. “Our common ancestor probably looked a lot like a tunicate larva.”
Not all evolutionary scientists agree with this: it is a controversial area of research. But Rothbächer and his colleagues have recently found evidence to support their ideas.
Their research revealed that the Hmx genes, which code for a pair of neurons in the tail of a tunicate tadpole, are related to genes that code for clusters of neurons in the head of a lamprey.
Lampreys are considered “living fossils” because they have been around for so long without their species changing. These marine animals are among the first vertebrates and look a bit like eels.
The evolutionary leap from tunicate life to lamprey life was large, but the Hmx gene appears to have crossed the dividing line. Its effect is slightly different in vertebrates.
When splicing Hmx genes from a lamprey into a species of tunicate called Intestinal Cionathe researchers found that the gene contributed to the expression of tail bipolar neurons.
In lampreys, however, the same genes contributed to the expression of sensory neurons in the skull.
Despite impacting nerves in different parts of the body, the similar function of Hmx genes in lampreys and tunicates suggests that they have a common evolutionary origin and may have played a role in centralization of the nervous system.
“Hmx has been shown to be a central gene that has been conserved during evolution,” says zoologist Alessandro Pennati, also from the University of Innsbruck.
“It retained its original function and structure and was probably found in this form in the common ancestor of vertebrates and tunicates.”
The findings suggest that vertebrate brains may have been recycled from the apparatus of their ancestors millions of years ago. And now here we are.
The study was published in Nature.
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