Professor Steven Stanley of Johns Hopkins University thinks that when it comes to evolution, the brain cell is underrated.
Since the 1960s, scientists have attributed an explosion in animal life 600 million years ago to rising levels of oxygen in the atmosphere. But Dr. Stanley says the abrupt appearance of hordes of complex animals among simpler life forms may have been triggered by a clever biological innovation: neurons, the highly specialized cells that form the brain and nervous system.
"Nothing much happened until the neuron appeared," he said yesterday. "Then it appears in all animal groups above that level and there's a big expansion" in animal body forms. Dr. Stanley, who studies ancient life forms, outlined his views earlier this week at a meeting of the Geological Society of America.
Before neurons evolved, Dr. Stanley said, multi-celled animals, such as sponges, lacked a command-and-control system needed to manipulate muscles and limbs. These simple creatures moved slowly or not at all.
"You couldn't have any kind of advanced feeding that involved manipulating food, catching food or breaking it apart," he said.
When neurons appeared, he said, organisms developed powerful new tools for exploiting their environments. The result was the proliferation of new life forms known as the Cambrian explosion.
Bruce Runnegar, a paleobiologist at the University of California at Los Angeles, said the emergence of neurons "obviously would be one of the key components of the story." But, he cautioned, "the Cambrian explosion is a complicated phenomenon, it has lots of different causes and triggers."
Dr. Runnegar said that collagen, the protein that makes up the connective fibers in tissue, bone and cartilage, may have been the crucial first step in the development of higher organisms.
"You need structural components first, and the other things second," he said.
But Dr. Stanley said that collagen-like materials were present in very simple animals long before more complex life forms evolved.
The Hopkins scientist said he began rethinking the Cambrian explosion after recent studies of soils and limestone suggested atmospheric oxygen levels almost 2 billion years ago were high enough to support complex animal life.
So Dr. Stanley started looking for a biological trigger, seeking the key difference between higher and lower life forms.
"What was not really present in single-celled organisms was the neuron," he said. "The interesting thing about neurons was basically it is the same kind of cell in a jellyfish and in the human brain. There is only one kind of neuron in the animal world."
The cell's uniform structure and unusual design, Dr. Stanley said, suggests the neuron developed in an animal that became the ancestor of all higher organisms.