A porcelain crab from the Atlantic Coast of the U.S. (Courtesy Liz Baird, NOAA)

A new study claims to clarify a longtime debate among biologists: does evolution, the gradual change of life forms, have any long-term trends? For animals, the answer is yes, the study suggests. Some scientists have suspected as much for a long time, but others have been skeptical. In the new research, scientists with the University of Bath, U.K., the University of Waterloo in Canada and Imperial College London studied fossils of crustaceans—crabs, lobsters and their relatives. Looking back 550 million years, they sought cases of crustaceans that evolved to become simpler than their ancestors. There were hardly any cases of this, the researchers said: most organisms evolved with increasingly complex structures, suggesting some mechanism drives change in this direction. “If you start with the simplest possible animal body, then there’s only one direction to evolve in—you have to become more complex,” said Matthew Wills of the University of Bath. But “sooner or later, however, you reach a level of complexity where it’s possible to go backwards.” Strangely, though, “hardly any crustaceans have taken this backwards route. Instead, almost all branches have evolved in the same direction, becoming more complex in parallel. This is the nearest thing to a pervasive evolutionary rule that’s been found.” Evolution occurs when some genes spread widely in a population spread more widely than others through the group. That normally occurs because certain genes are more beneficial than others to their bearers; as a result they reproduce more. Ceaseless repetitions of this process leads whole species to change and become new species, forming a sort of family tree of species. A seemingly obvious feature of this tree is that complexity constantly increases; the human evolution from apes is the paradigmic example. But some have disputed that this reflects some overall trend. The late Harvard biologist Stephen Jay Gould argued that the assumption of ever-increasing complexity is an illusion, arising from the fact that life started with the simplest forms—as it almost had to. Life could only get more complex from there. But this doesn’t rule out that more elaborate forms can randomly fluctuate upward and downward in complexity, he added. Wills and colleagues, however, found that the downward slide is rare. In their study, the only animals that evolved from complex to simpler forms were either “parasites, or animals living in remote habitats such as isolated marine caves,” Wills said. The driving trend toward complexity affected all “free-living animals in the ‘rat-race’ of evolution,” he added. “It seems that competition may be the driving force behind the trend. Evolution “looks far more like a disciplined march than a milling crowd,” he added. However, Wills colleague Sarah Adamowicz of the University of Waterloo added that the findings don’t necessarily apply to all organisms. “We must not forget that bacteria—very simple organisms—are among the most successful living things. Therefore, the trend towards complexity is compelling but does not describe the history of all life.”