First there was one: Seeking the Roots of Multicellularity
Life used to be so simple.
Back more than a billion years ago, it consisted of just one cell. It might have had had a nucleus – or not. It was absolutely microscopic.
And then, over time, life became more complex. And its complexity was not as simple as one might think.
“There are multiple ways of being multicellular,” said Fred Spiegel, professor of biological sciences in the J. William Fulbright College of Arts and Sciences. Spiegel, in addition to being a multicellular animal known as a human, studies slime molds, fungi and a group of microorganisms known as protists. His work has led him to contemplate questions of how life forms relate to one another. One day recently, he answered questions about the origins of multicellularity.
Multicellularity occurs in many different ways in nature. In animals, a fertilized egg goes through embryonic development. In the same biological group as animals, some fungi form complex multicellular structures like mushrooms and other tissues. Even within the fungi, multicellularity happens in many different ways: some fungi live as individuals but come together cooperatively to form a multicellular mass called a fruiting body.
“It’s unlikely that there was ever one thing that you could call multicellularity,” Spiegel said. Its origins remain obscure, but the sheer prevalence of it in different forms suggests its success.
While multicellular organisms have succeeded in surviving, an even greater variety of single-celled organisms still thrive. Spiegel sees a human-centric drive towards the study of large multicellular organisms, with plants and animals at the top and single-celled creatures near the bottom of the interest spectrum, despite the fact that our own body mass contains more single-celled organisms than human cells. Your body plays host to around 100 trillion bacteria, as well as fungi and single celled anaerobic organisms called archaea. At this point, scientists know very little about the microscopic creatures that inhabit our bodies.
“We’re big, we’re complex, we’re multicellular, and therefore we think we must be important” in the biological scheme of things, Spiegel said. “But you’ve got to take the pride out of multicellularity to see what it means.”
Scientists seek to know more about the origins of multicellular organisms by examining the relationships between the organisms and building “trees” that show their relationships, then mapping the characteristics of these organisms on the trees and refining the relationships between them. Spiegel and his colleagues have shown that multicellularity has evolved independently in different groups, including a species in a group where it previously was not known before.
In the most widely distributed multicellular lifestyle, practiced by cellular slime molds, individual cells come together to form a complex “fruiting body.” However, this innovation occurs in organisms besides slime molds, including an amoeba called Guttulinopsis vulgaris studied by biologist Jeff Silberman and former graduate student Matt Brown. This amoeba represents the only known species in its particular branch of the tree of life that exhibits this form of multicelluarity, offering yet another clue to how multicellular organisms evolved. This clue suggests that figuring out the origin of multicellular organisms grows ever more complex.
The problem is that these trees can only go so far back. Scientists can look at certain structures and processes that organisms have in common, such as mulitcellularity or sex, to determine what the last common ancestor would look like. For instance, consider eukaryotes, which are organisms with cells that contain complex structures in membranes. Since all eukaryotes reproduce sexually, then it seems likely that the last common ancestor of eukaryotes reproduced through sex. However, because biologists can’t know what processes or structures might have existed in extinct organisms, they can’t be certain of the exact origins of a particular trait, such as becoming multicellular.
“We can figure out what the last common ancestor of a certain lineage might look like, but we don’t know what was there before that,” Spiegel said. “We don’t know how something got to the point of being the last known common ancestor.”
Scientists do know, however, that multicellular processes evolved separately in different organisms at different times.
“The ‘final products’ – animals, plants, fungi and amoebae, among others – indicate independent origins” for multicellularity, Spiegel said.
Also, multicellular organisms do have some similarities, in that chemical signals bring cells together to create life in its different forms. You can also see this responsiveness to the environment in unicellular organisms.
“Unicellular creatures will go towards food or light, and move away from noxious substances,” Spiegel said.
The origins of multicellularity will remain obscure far into the future. In the meantime, scientists like Spiegel, a mycologist specializing in slime molds, will continue to slowly piece the picture together. Spiegel is an advocate for the unseen, a champion of an under-examined world of biology that exists outside of animals, plants and pathogenic microorganisms. Some day he hopes to write a basic biology textbook that emphasizes the microscopic world.
As he sees it, “Mammals are just one small example of the different ways you can be alive.”
Sex and the single cell
By Melissa Lutz Blouin
“All you mycologists ever think about is sex,” commented one of biologist Fred Spiegel’s students after taking one of his classes. Spiegel wishes that more biologists thought about sex, but in a universal way.
“Animals are poor models for understanding the universal aspects of sex,” Spiegel said in a commentary for the Proceedings of the Royal Society. Plants have sex. Insects have sex. Fungi have sex. What sex boils down to, according to scientists, is a special type of cell division that creates the reproductive cells known as sperm and egg cells in plants and animals. Following that, the reproductive cells from two cell nuclei come together and fuse, which is fertilization. That’s all you need.
Sex only occurs in eukaryotes, organisms with cells that contain complex structures enclosed in membranes. Scientists don’t know if sex happens in all species in this domain, however.
“The absence of the observation of sex is not the same as the absence of sex,” Spiegel said. “If one learns to ‘think about sex all the time,’ one may be more likely to find it.”
Spiegel cites slime molds, also known as fruiting amoebae, as an example. Once biologists carefully examined the life cycles of certain types of slime molds, they found evidence of sex.
Organisms that reproduce asexually may also offer clues to those that reproduce using sex – especially if some strains of those organisms do both. In one major group of amoebae, known as plasmodial slime molds, one set reproduces using sex and the other does it without. Looking carefully at these biological processes could help scientists learn to recognize sex when they see it elsewhere.
“If we really want to think critically about sex in the most general sense, amoebae can tell us a lot that we can use to develop those generalizations,” Spiegel said. “They can relieve us of the animal-based biases about sex.”
“Looking carefully at these biological processes could help scientists learn to recognize sex when they see it elsewhere.”
Choanoflagellates are a group of free-living unicellular organisms that can also form colonies. Some scientists think they are the closest living relatives of animals. Scientists like Fred Spiegel believe that multicellularity, found in plants, fungi and humans, evolved multiple times in different ways.