The biophysicist Manu Prakash vividly remembers the moment, late one night in a colleague’s laboratory a dozen years ago, when he peered into a microscope and met his new obsession. The animal beneath the lenses wasn’t much to look at, resembling an amoeba more than anything else: a flattened multicellular blob, only 20 microns thick and a few millimeters across, with neither head nor tail. It moved on thousands of cilia that blanketed its underside to form the “sticky hairy plate” that inspired its Latin name, Trichoplax adhaerens.
This odd marine creature, classified as a placozoan, has practically an entire branch on the evolutionary tree of life to itself, as well as the smallest known genome in the animal kingdom. But what intrigued Prakash most was the well-orchestrated grace, agility and efficiency with which the thousands to millions of cells in Trichoplax moved.
After all, such coordination usually requires neurons and muscles — and Trichoplax has neither.
Prakash later teamed up with Matthew Storm Bull, then his graduate student at Stanford University, to make this strange organism the star of an ambitious project aimed at understanding how neuromuscular systems might have evolved — and how early multicellular creatures managed to move, find food and reproduce before neurons existed.
“I often jokingly call this neuroscience without neurons,” Prakash said.
In a trio of preprints totaling more than 100 pages — posted simultaneously on the arxiv.org server last year — he and Bull showed that the behavior of Trichoplax could be described entirely in the language of physics and dynamical systems. Mechanical interactions that began at the level of a single cilium, and then multiplied over millions of cells and extended to higher levels of structure, fully explained the coordinated locomotion of the entire animal. The organism doesn’t “choose” what to do. Instead, the horde of individual cilia simply moves — and the animal as a whole performs as though it is being directed by a nervous system. The researchers even showed that the cilia’s dynamics exhibit properties that are commonly seen as distinctive hallmarks of neurons.
The work not only demonstrates how simple mechanical interactions can generate incredible complexity, but also tells a compelling story about what might have predated the evolution of the nervous system.
“It’s a tour de force of biophysics,” said Orit Peleg of the University of Colorado, Boulder, who was not involved in the studies. The findings have already started to inspire the design of mechanical machines and robots, and perhaps even a new way of thinking about the role of nervous systems in animal behavior. [Continue reading…]