The evolution of the human brain

99.99 percent of species don’t have a brain like we have.

But as Dr. Rob DeSalle, curator in the American Museum of Natural History’s division of invertebrate zoology, explained at an event at the museum last night, studying organisms without brains is one of the best ways to study the human brain.

DeSalle moved through the Tree of Life—a diagram of species and their connections to one another—to describe how the brain and brain-like structures evolved.

Bacteria, for example, use chemicals to respond to the presence of other single-celled bacteria. They in effect act as a multicellular unit through this communication—and they don’t have a brain or a nervous system.

Social slime molds—or dictyostelium—take this ability one step further. They live in soil as single-celled, amoeba-like organisms. But they group together when faced with starvation or other adverse conditions, forming worm-like creatures capable of locomotion. Even without brains, slime molds make decisions. One species of slime mold, physarum, is known for its ability to efficiently find the quickest way through a maze. In addition, researchers have shown that dichtyostelium can carry the same mitochondrial mutations known to wreak havoc in the human nervous system.

Sponges have half of the genes required for synapses. And jellyfish, coral, and others members of the phylum cnidaria have neural nets—a series of cells that communicate through basic synapses. While not having a true brain, nematodes have a nervous system that forms a ring around their digestive system, a ganglia (a group of nerve cells), and the ability to make decisions based on smell.

Once we get to fish and lizards, the brain as we know it begins to take shape. These animals have a basic cortex, cerebellum, and basal ganglia. In mammals, an organizational change took place, with genes expressed in higher amounts, altering the size of certain brain structures.

As DeSalle described, human brains are most likely the result of genetic drift (chance changes in the gene pool of a small population), natural selection, and, mostly, luck. “Our brains are an inelegant solution to a problem,” concluded DeSalle. “They are part of an evolutionary heritage.”

–Johanna Goldberg

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