Charles Darwin shocked the world when he
talked about the “lowly origins” of humanity, and he was just talking apes.
Nick Strausfeld, in the first of the UA College of Science “Evolving Brain” lectures, traces development of the human brain to even lower creatures — the unfound, and perhaps unfindable, common ancestor of humans, flatworms and just about everything else, more than half a billion years ago.
Strausfeld, director of the Center for Insect Science and a Regents’ Professor of neuroscience at the University of Arizona, says there is emerging and persuasive evidence that the basic circuitry of our brains evolved in lowly sea creatures, early in the evolution of life on Earth.
The work of Strausfeld and others in the field trod different paths to arrive at the conclusion, searching the fossil record for the earliest evidence of neural development and comparing taxonomic traits and genetic material for similarities between such diverse creatures as human beings and dung beetles.
The dung beetle will be a central character in Strausfeld’s talk Monday: “Time Traveling: What Our Brains Share With Beetle Brains.”
“If you compare our behavior with that of the dung beetle, you find some very interesting similarities,” Strausfeld said in an interview last week.
The dung beetle, in rolling its ball of fecal matter, transporting it, burying it and finding it again demonstrates decision-making, memory and selection of action, Strausfeld said.
Those three aspects of behavior are, “in a sense, identical to ours,” he said.
One of the questions Strausfeld explores in his research is whether those brain functions evolved separately in living creatures that have brains, or whether they were inherited from a common ancestor.
He favors the latter, “homology” scenario, which, he said, is shared by many evolutionary biologists.
“If you ask most neuroscientists, they would say ‘no’ very emphatically.”
They would favor “convergence,” a scenario in which these similar brain structures evolved separately and just happen to have such similar traits.
One thing is indisputable in the fossil record, Strausfeld said. Complex brain structures evolved long ago.
Strausfeld and his colleagues have studied fossils uncovered from the Chengjiang formation in Yunnan province in southwest China, which formed 500 million to 520 million years ago.
A fossilized marine arthropod known as a megacheiran had a nervous system similar to those of contemporary horseshoe crabs and scorpions. It is the oldest example of a complete nervous system ever found.
Discovery of the megacheiran or “mega-claw,” brain was described by Strausfeld and colleagues in London, China and Japan in an October issue of the journal Nature.
“These are pretty spectacular creatures,” Strausfeld said, with a fore-brain, mid-brain and hind-brain.
He does not study vertebrates, but he says he has seen similarities in ancient vertebrate fossils. Strausfeld said he clearly sees brain structure with three-segment organization in published illustrations.
Looking further back for common ancestors may not be possible, he said. The conditions for tissue fossilization in the sediment of seabeds did not exist more than 540 million or so years ago.
There is other evidence.
Studies have found homologous segmental gene expression in the brains of creatures as divergent as flies and mice, he said.
The technique of cladistics, which catalogs similar and dissimilar traits across numerous living species, also supports the idea of brain homology, he said.
The big argument raised against a common ancestor for all creatures with brains is the lack of brain structure in many contemporary species.
Strausfeld said numerous species lose brain function when it is no longer necessary. Old barnacles, for example, have an active larval life, but when they settle into an ecological niche that doesn’t require movement, they lose their energy-sapping nervous system.
“Complexity can give rise to simplicity” through the process of evolution, Strausfeld said.
Strausfeld is convinced of the rightness of his position. He even chose the moniker “flybrain” for his email address. But he cautions that “the history of science is full of proofs that have been disproved.”