Researchers Rethink the Ancestry of Complex Cells
New studies revise ideas about the symbiosis that gave mitochondria to cells and about whether the last common ancestor of all eukaryotes was one cell or many.
Research into the origins of the complex cells called eukaryotes typically tries to trace lineages of the organisms back to a single ancestral cell. But now scientists are considering whether that common “ancestor” might really be an entire population of diverse cells.
Our planet formed a little over 4.5 billion years ago, and if the most recent estimates are correct, it wasn’t long before life arose. Not much is known about how that happened because it’s maddeningly difficult to investigate. It’s also proved tough to study what happened next, during the first billions of years of evolution that followed, when the main domains of life emerged.
A particularly vexing mystery is the rise of the eukaryotes, cells with well-defined internal compartments, or organelles, which are present only in animals, plants, fungi and some microbes like protists — our evolutionary kin. The earliest eukaryotes left no clear fossils as clues, so researchers are forced to deduce what they were like by comparing the structural and molecular details of later ones and inferring their evolutionary relationships.
Right now is “an incredibly exciting time” for such research, said Michelle Leger, a postdoctoral fellow at the Institute of Evolutionary Biology in Barcelona, Spain. With modern genetic sequencing technologies, scientists can read the entire genomes of diverse life forms, and as microbial life is revealed in ever-increasing detail, new species and other taxonomic groups are coming to light. With that wealth of data, researchers are tracing lineages of organisms backward through time. “We’re trying to approach the problem from so many sides,” she said. “That’s pushing us closer to the first eukaryotes.”
This paramecium, a single-cell protozoan microbe, has a nucleus, mitochondria and other organelles that are hallmarks of eukaryotic cells.
Michael Abbey/Science Source
And those first eukaryotes may depart significantly from what most scientists expected, if some recent findings are any indication. Earlier this month, one team presented evidence that a signature event in eukaryote evolution — the development of the organelles called mitochondria — might have unfolded quite differently than was theorized. Meanwhile, other researchers have suggested that the earliest “ancestor” of all eukaryotes might not have been a single cell at all, but rather a mixed population of cells that avidly swapped DNA. The difference is subtle, but it might be important for understanding the evolution and diversity of the eukaryotes we see today.
The Ancestral Eukaryotes
The very first cells — the first life forms on this planet — were prokaryotes, but they were not all alike. Even early on, two very distinct lineages emerged, the archaea and the bacteria. The archaea might have been the first to thrive because even now they can survive in extreme environments like hot vents and super-saline pools. But it’s also possible that archaea and bacteria split from the first cells at the same time and began to diversify independently from the start. Figuring out definitively when and how the split occurred is probably impossible given how much time has passed; fossil evidence is nonexistent, and organisms from both branches have swapped genes extensively through horizontal gene transfer (as opposed to the “vertical” transfer of genes down through generations), which complicates analyses of their genomic history.
What we do know is that the story of eukaryotes began when some rogue archaeal cell split from the rest and founded what was long considered an entirely new domain of life. “First and fundamentally we are a very strange kind of archaea,” said Maureen O’Malley, a philosopher of biology affiliated with the University of Bordeaux and the University of Sydney.
...
Read more here: Quanta Magazine
