Mitochondria and the origin of eukaryotes
Were the powerhouse organelles a driving force or a late addition in the evolution of more complex cells like ours?
By Viviane Callier 06.08.2022
For billions of years after the origin of life, the only living things on Earth were tiny, primitive cells resembling 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 a different one — a bacterium.
Instead of being digested, the bacterium took up permanent residence within the other organism as what biologists call an endosymbiont. Eventually, it integrated fully into its archaeal host cell, becoming what we know today as the mitochondrion, the crucial energy-producing component of the cell.
Its acquisition has long been viewed as the key 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 sophisticated cells of higher organisms, or eukaryotes, including ourselves.
It’s a neat story you’ll find in most biology textbooks — but is it quite that simple? In the last few years, new evidence has challenged the notion that mitochondria played a seminal role in this transition. Researchers sequencing the genomes of modern-day relatives of the first eukaryotes have found many unexpected genes that don’t seem to come from either the host or the endosymbiont. And that, some scientists suggest, might mean that the evolution of the first eukaryotes involved more than two partners and happened more gradually than suspected.
Others don’t see a reason yet to abandon the theory that the acquisition of the mitochondrion was the spark that ignited the rapid evolution of eukaryotes — giving rise, eons later, to plants, animals, vertebrates, people. Fresh evidence from genomics and cell biology may help resolve the debate, while also pointing to knowledge gaps that still need to be filled to understand one of the foundational events in our own ancestry, the origin of complex cells.
