A cartoon image of mitochondria as green, oblong shapes on a black background.

After the origin of life, the only living things on Earth were tiny, primitive cells. More than 1.5 billion years ago, a primitive cell of the archaea swallowed a bacterium.

The bacterium took up permanent residence in the other organisms instead of being eaten. The crucial energy-production component of the cell was integrated fully into its archaeal host cell.

The most important evolutionary leap since the origin of life is the transition from early primitive cells to the more sophisticated cells of higher organisms.

You will find a neat story in most biology textbooks. In the last few years, new evidence has challenged the idea that mitochondria played a major part in this transition. Many genes that don't seem to come from either the host or the endosymbiont have been found by researchers. Some scientists think that the evolution of the first eukaryotes may have been more gradual than thought.

Some people don't see a reason to abandon the idea that the acquisition of the Mitochondrion sparked the rapid evolution of the organisms. Fresh evidence from cell biology may help resolve the debate, as well as pointing to knowledge gaps that still need to be filled to understand the origin of complex cells.

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Prokaryotic cells—modern-day bacteria and archaea—tend to be small and simple, with few internal structures. Eukaryotic cells such as those of modern-day plants and animals are much more sophisticated. They have many internal structures, or organelles, that carry out specific functions.
Prokaryotic cells—modern-day bacteria and archaea—tend to be small and simple, with few internal structures. Eukaryotic cells such as those of modern-day plants and animals are much more sophisticated. They have many internal structures, or organelles, that carry out specific functions.

Mysterious extras

The mystery genes turned up in the last decade when researchers, including Gabaldn, an evolutionary genomicist at the Barcelona Supercomputing Center, and his colleagues took advantage of today's cheap gene-sequencing technology

They were expecting to find genes that came from either the archaeal host or the alphaproteobacteria. The genes that were found were from a wide range of otherbacteria.

According to Gabaldn and colleagues, the cellular ancestors of eukaryotes had acquired genes from a number of partners. The partners could have been other free-livingbacteria that passed some of their genes to the ancestral host. They suggested that the tango that led to eukaryotes involved more than one dancer.

Gabaldn said that there are additional contributions from additional partners.

It's difficult to know where those ancient foreign genes came from. There are many more recent, looser endosymbioses where the origin of foreign genes is easier to identify. He says that studying these might give us a chance to understand how the first genes could have evolved.

Eukaryotes arose from primitive cells known as archaea that eventually acquired complex traits such as internal, membrane-bounded structures called organelles—but the exact sequence of events is poorly understood, and many species presumed to be intermediate stages (X) are now extinct. In particular, scientists are still debating whether the ancient symbiosis that led to mitochondria occurred early in the process—suggesting that it was a key trigger—or was a later refinement.
Eukaryotes arose from primitive cells known as archaea that eventually acquired complex traits such as internal, membrane-bounded structures called organelles—but the exact sequence of events is poorly understood, and many species presumed to be intermediate stages (X) are now extinct. In particular, scientists are still debating whether the ancient symbiosis that led to mitochondria occurred early in the process—suggesting that it was a key trigger—or was a later refinement.
Earth were tinythings on Earthprimitive cells resemblingeukaryotes
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