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.
