If you go far enough back along the branch of the evolutionary tree of life that humans sit on, you get to the part near the trunk where verterbrates (creatures with spines) split from invertebrates (creatures without spines). Current theories suggest the complex brain we share with our vertebrate relatives appeared after this point, but now, thanks to a marine worm with a proboscis that burrows into sand on the sea floor, a new study from the US is challenging that view.
Our brain is much older than we think, suggest researchers from Stanford and the University of Chicago, who write about their findings in the 14 March online issue of Nature.
Study author Chris Lowe, an evolutionary biologist at Stanford’s Hopkins Marine Station in Pacific Grove, California, told the press:
“This paper will change the way people think about brain evolution.”
One of the early stages of brain development in the vertebrate embryo, is the emergence of a genetic “scaffold” of signaling centers that control how chemical signals are sent in the developing brain. The current thinking is that important elements of this scaffold are only to be found in vertebrates such as humans, since you don’t see them in other creatures, even close relatives of vertebrates.
But, if you cast your net wide enough, you do, apparently, find evidence, of essential genetic machinery thought to be exclusive to vertebrates, in other creatures. Lowe and colleagues found evidence of these chemical signaling centers in the most surprising place: a a sea dwelling, bottom-feeding acorn worm called Saccoglossus kowalevskii.
The brains of these marine worms aren’t like those of vertebrates, and in fact they began to evolve separately from vertebrates over 500 million years ago. They even belong to a different phylum, the hemichordates. (A phylum is a branch near the trunk of the tree of life: the rank order is Life (the trunk), Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species.)
Lowe, an assistant professor of biology, said:
“The closer we looked, the more similarities we found between these strange worms and vertebrate brains in their underlying molecular blueprints.”
“This suggests that essential parts of these blueprints, previously thought to be unique to complex brains, have much earlier evolutionary origins,” he added.
Lowe and colleagues say their findings show we need to look more widely, and start exploring under-investigated and different- looking animals, to see the whole picture of how we all evolved. We need to consider that modern animals may have lost certain ancient processes and traits, they say.
We may see more such discoveries, because as evolutionary biology matures as a field, it brings new biomedical technologies that look at the tree of life in a totally different way: beyond the surface of living creatures and their form, to the genetic blueprints that ultimately control the growth of anatomical structures.
For instance, using such tools, Lowe and colleagues previously found that many important genes in acorn worm embryos were active in similar parts of the body as in mice and other vertebrates, despite their being anatomically quite different.
For this study, they looked at brain origins and searched for the molecular signature of three vertebrate brain-signaling centers that provide a framework for organizing proteins and cells.
By tagging proteins with markers, they found that embryonic acorn worms expressed three protein combinations in comparable regions of the embryo as vertebrates during the development of their three-part brains.
In vertebrate embryos, the brain-signaling centers control where the forebrain, midbrain and hindbrain will form in later stages of development.
In acorn worm embryos, there is a forebrain-like signaling centre that controls the proboscis, another midbrain-like one that controls a collar of tissue just below the proboscis, and a hindbrain-like one near where the collar meets the worm’s trunk.
And Lowe and colleagues also found that the proteins in each signaling center communicate with each other in much the same way as they do in vertebrates. The main difference is the form that the brain takes in the worm: it never actually comes together as a distinct brain. Instead, nerve cells are dotted all over the worm’s proboscis and collar, forming what Lowe describes as a “skin brain”.
Biologists had thought that signaling centers had first come together in vertebrates because they did not see them in sea squirts or lancelets, which are actually closer relatives to vertebrates than the acorn worm.
Lowe and colleagues suggest their new findings show that lancelets have lost many of those genetic processes, even though they possesses a rather complex central nervous system more similar to vertebrates than their own part of the tree of life.
This was one of the biggest surprises to them, said Lowe, everyone looks at lancelets as a sort of “living fossil” proof of how the vertebrate brain evolved.
“These findings remind us that modern animals are all at the ‘tips of the branches’ of the evolutionary tree,” said Lowe.
“And when we are searching for evidence about what our common ancestors were like, we have to look at all the branches to find the right clues,” he added.
Written by Catharine Paddock PhD