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The evolution of a head has been traced surprisingly far down our ancestral line

What’s in a head? According to new research, a little bit of our ancestors’ tails.

In the early days of complex, multicellular life on Earth, animals started out without spines or brains. They just had a network of neurons scattered throughout their bodies. Over millions of years, however, that system somehow became concentrated on one side. But how?

Tunicates, or “sea pipes,” are the closest living relatives of vertebrates, and they don’t have a true head.

Their central nervous system is instead made up of tufts of neurons in the anterior and posterior parts of their bodies, with a dorsal strand connecting them both. As adults, these animals look like stationary spongy blobs, with no apparent head or tail. But as tadpole-like larvae, their large brains are easier to distinguish.

“Tunisonal animals are like an evolutionary prototype for vertebrates,” explains zoologist Ute Rothbächer of the University of Innsbruck in Austria. “Our common ancestor probably looked very similar to a tunicate larva.”

Low resolution BTN Ciona 2.jpgPIC tunicate tadpole with bipolar tail neurons (green). (The University of Innsbruck)

Not all evolutionary scientists agree: It’s a controversial area of ​​research. But Rothbächer and his colleagues recently found evidence to support their ideas.

Their research has shown that Hmx genes, which code for a pair of neurons in a tunicate’s tail, are related to the genes that code for tufts of neurons in a lamprey’s head.

Lampreys are considered “living fossils” because they have been around for so long with little change in their species. These sea creatures are some of the first vertebrates and they look like eels.

The evolutionary leap from tunicate life to lamprey life was a big one, but the Hmx gene seems to have crossed the gap. The effect is slightly different in vertebrates.

When splicing the Hmx genes of a lamprey in a tunicate species called Ciona intestinalisresearchers found that the gene helped boost the expression of bipolar tail neurons.

When pricked, however, the same genes helped drive the expression of sensory neurons in the skull.

Despite affecting nerves in different parts of the body, the similar function of Hmx genes in lampreys and tunicates suggests they share 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 preserved throughout evolution,” said zoologist Alessandro Pennati, also of the University of Innsbruck.

“It has 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 their ancestors’ apparatus millions of years ago. And now, here we are.

The study is published in Nature

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