One of the most fundamental question evolutionary biologists are trying to answer is how did multicellular life evolve from single celled organisms. Researchers from New Zealand, Germany and the USA believe they have found a counter-intuitive hint after studying organisms evolve in real time: cheating, non-cooperative cells may have pressured evolution to work on a program that would integrate two cell states.
Cheating in life
Cheats are cells that do not contribute to the integrity of the group, but still take advantage of the benefits of being part of a collective. Sounds familiar? Every society has its cheats, and like in the human world, if there are too many cheats in cellular group, then the group collapses. If a group works together, but some decide to slack, it’s not the end of the world, but if everybody slacks society tumbles.
“Cheats are typically viewed as the greatest impediment to the emergence of multicellular life because they collapse cooperating groups—the obvious thing to do is to get rid of them,” said Lead researcher Distinguished Professor Paul Rainey from the New Zealand Institute for Advanced Study (NZIAS)
This wasn’t that obvious, though, so Caroline Rose and Katrin Hammerschmidt from the New Zealand Institute for Advanced Study (NZIAS) tested cell lineages for five years to see whether cheats had any contribution to their evolutionary makeup. Lineages were thus constructed where the cheats were embraced or rejected, beginning with single cells then showing how simple cooperating groups of bacteria can reproduce under various life cycles.
“When cheats were embraced we discovered something surprising,” Dr Rose says. “Evolution saw a new kind of entity—a group comprised of two different cell states: cheating and cooperating cells. Evolution couldn’t focus on just one state or the other; for lineages to persist, evolution had to see both types—it had to work on a developmental programme.”
Dr Hammerschmidt explains: “When this happened, the groups became better adapted, but they did so at the expense of the individual cells that made up the groups. This might seem nonsensical, but it is precisely what is thought to happen during major evolutionary transitions: the higher (group) level subsumes the lower (cell) level, with the lower level eventually coming to work for the good of the collective. Nothing so remarkable happened when we performed the same experiment, but with a life cycle in which we got rid of cheats.”
When cheaters were embraced in the life cycle, the life cycle alternated between phenotypic states and selection fostered a development mechanism that bred collectives with a decoupled fitness from the fitness of constituent cells. Biologists use the word fitness to describe how good a particular genotype is at leaving offspring in the next generation relative to how good other genotypes are at it. So if brown beetles consistently leave more offspring than green beetles because of their color, you’d say that the brown beetles had a higher fitness.
“Little is known”, explains Professor Rainey, “but life cycles involving at least two different states are almost universal in the world of multicellular organisms. I suspect that this is because multiphase life cycles generate an organismal configuration that delivers to natural selection a machine-like entity with which it can really work.
“The emergence of these primordial life cycles holds the key to understanding some of biology’s most profound problems: the origins of multicellularity; the origins of soma/germ differentiation, of reproduction, of development—even the origins of cancer.”
The researchers are still experiment, but so far they believe they have stumbled across a key insight that might explain how multicellular life appeared. Findings appeared in Nature.