The Evolution of a Head Has Been Traced Back Surprisingly Far Up Our Ancestral Line

What goes on within the mind? A little portion of our ancestors' tails, according to recent studies.

Animals didn't have spines or brains when complex, multicellular life first appeared on Earth. They only have a neuronal network that covered their entire body. But somehow, over the span of millions of years, that system got focused on one end. Yet how?

The closest living cousins of vertebrates are tunicates, sometimes known as "sea squirts," which lack a real head.

Instead, the anterior and posterior regions of their bodies contain clusters of neurons that make up their central nervous system, and a dorsal strand connects them both. Adult forms of these creatures resemble stagnant sponge-like blobs without a distinct head or tail. The cerebrum of these larvae, resembling tadpoles, is easy to distinguish.

According to scientist Ute Rothbächer of the University of Innsbruck in Austria, tunicates are similar to an evolutionary precursor for vertebrates. "A tunicate larva was probably very similar to our common ancestor."

It's an area of inquiry that not all evolutionary experts agree on. But Rothbächer and his associates have just discovered proof to back up their theories.

They discovered that the genes that encode for clusters of neurons in a lamprey's head are connected to the Hmx genes, which encode for a pair of neurons in a tunicate tadpole's tail.

Because they have been present for such a long time with few if any alterations to their species, lampreys are referred to as "living fossils." These sea creatures, which resemble eels somewhat, are among the first vertebrates.

The transition from tunicate life to lamprey life required a significant evolutionary leap, yet the Hmx gene appears to have survived it. In vertebrates, it has a little different impact.

Researchers discovered the Hmx gene contributed to the production of bipolar tail neurons when it was spliced into a tunicate species called Ciona intestinalis.

The same genes, however, also aided in the production of sensory neurons in the skull in lampreys.

The comparable function of Hmx genes in lampreys and tunicates shows they share a common evolutionary origin and may have contributed to the centralization of the nervous system despite affecting nerves in various areas of the body.

According to biologist Alessandro Pennati of the University of Innsbruck, "Hmx has been shown to be a central gene that has been conserved across evolution."

It was most likely discovered in this form in the common ancestor of vertebrates and tunicates and has maintained its original function and structure.

According to the research, vertebrate brains may have previously been recycled from their predecessors' equipment millions of years ago. And here we are right now.

The study was published in Nature.