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Drive to complexity seen in animal evolution

March 17, 2008
Courtesy University of Bath
and World Science staff

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: they tend to be­come more com­plex. Some sci­en­tists have sus­pected as much, but oth­ers have been skep­ti­cal.

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


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, “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 in a popula­t­ion spread more widely than oth­ers through the group. That nor­mally oc­curs be­cause cer­tain genes are more ad­vanta­geous than oth­ers; as a re­sult their bear­ers are able to 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 most famous ex­am­ple. 

But some have dis­put­ed that such an over­all trend exists. 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 spe­cies that be­came sim­pler 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... [it] looks far more like a dis­ci­plined march than a mill­ing crowd.”

Wills col­league Sar­ah Adamow­icz of the Uni­ver­s­ity of Wa­ter­loo cau­tioned that the find­ings might not 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.”


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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.”