For billions of years after life originated, the only living things on Earth were small, primitive cells that resembled today’s bacteria. But then, more than 1.5 billion years ago, something remarkable happened: One of those primitive cells, belonging to a group known as the archaea, swallowed another — a bacterium.
Rather than being digested, the bacterium took a permanent residence in the other organism as what biologists call an endosymbiont. Eventually, it fully integrated into its archaeal host cell, becoming what we know today as the mitochondrion, the critical energy-producing component of the cell.
Its acquisition has long been viewed as the most important step in what is arguably the most important evolutionary leap since the origin of life itself: the transition from early primitive cells, or prokaryotes, to the more advanced cells of higher organisms, or eukaryotes, including ourselves. †
It’s a nice story you’ll find in most biology textbooks, but is it really that simple? In recent years, new evidence has challenged the idea that mitochondria played a pioneering role in this transition. Researchers who sequenced the genomes of modern-day relatives of the first eukaryotes have found many unexpected genes that do not appear to come from the host or the endosymbiont. And that, some scientists suggest, could mean that the evolution of the first eukaryotes involved more than two partners and was more gradual than suspected.
Others see no reason to abandon the theory that the acquisition of the mitochondrion was the spark that fueled the rapid evolution of eukaryotes — which gave rise to plants, animals, vertebrates and humans centuries later. New evidence from genomics and cell biology may help settle the debate, while also pointing to knowledge gaps that have yet to be filled to understand one of the fundamental events in our own ancestors, the origin of complex cells.
Prokaryotic cells – modern bacteria and archaea – are usually small and simple, with few internal structures. Eukaryotic cells like those of modern plants and animals are much more sophisticated. They have many internal structures, or organelles, that perform specific functions.
Mysterious extras
The mysterious genes surfaced over the past decade as researchers, including Toni Gabaldón, an evolutionary genomicist at the Barcelona Supercomputing Center, and his colleagues took advantage of today’s inexpensive gene sequencing technology to examine the genomes of a wide variety of eukaryotes. , including several obscure, primitive, contemporary relatives of early eukaryotes.
They expected to find genes whose lineage went back to either the archaeal host or the mitochondrial ancestor, a member of a group called the alphaproteobacteria. But to their surprise, the scientists also found genes that seemed to come from a host of other bacteria.
Gabaldón and colleagues hypothesized that the cellular ancestor of eukaryotes had obtained the genes from several partners. Those partners could be additional endosymbionts that were later lost, or free-living bacteria that passed one or some of their genes to the ancestral host in a common process called horizontal gene transfer. Regardless, the tango that led to eukaryotes involved more than two dancers, they suggested.
“It is now clear that there are additional contributions from additional partners,” said Gabaldón, who wrote about early evolution of eukaryotes in the 2021 Annual Review of Microbiology.
It’s hard to know exactly where those old strange genes came from, because so much time has passed. But there are many more recent, looser endosymbioses where the origin of foreign genes is easier to identify, John McCutcheon says.† an evolutionary cell biologist at Arizona State University in Tempe who wrote about endosymbiont evolution in the 2021 Annual Review of Cell and Developmental Biology. Studying these, by analogy, could give us a chance to understand how mitochondria and the first eukaryotes might have evolved, he says.
Eukaryotes arose from primitive cells known as archaea that eventually acquired complex properties such as internal, membrane-bound structures called organelles, but the exact sequence of events is poorly understood and many species are believed to be intermediate stages are (X), are now extinct. In particular, scientists are still debating whether the ancient symbiosis that led to mitochondria occurred early in the process — suggesting it was a major trigger — or a later refinement.
