杏吧原创

We may have imagined our most distant ancestor wrongly. The first complex cell may have been bigger and even more internally complex than anyone suspected 鈥� and that may have helped it survive.

Over a billion years ago, evolution gave rise to the first complex cell. It is often pictured as something like an amoeba or a white blood cell: a roughly spherical blob with a single packet of DNA at its core.

But a new study argues that is wrong. Instead, it says the first complex cell was much larger and didn鈥檛 have just one packet of DNA: it had several, perhaps dozens. Having multiple copies of its genes would have helped it survive and adapt, the researchers say.

鈥淚t鈥檚 a really interesting new idea,鈥� says at the University of Li猫ge in Belgium, who wasn鈥檛 involved in the study. Most researchers never even thought to question the number of DNA stores, she says.

Cells are integral to life. They are bubble-like receptacles that contain DNA, protein and the other vital molecules. Most living cells have relatively simple internal structures and fall into one of two groups: bacteria and archaea. But some 鈥� a group called eukaryotes 鈥� have bigger and more intricate cells. A eukaryotic cell has a central nucleus where it keeps its DNA, and sausage-shaped bodies called mitochondria that supply it with energy.

All animals, plants and fungi are eukaryotes, making the event that was the origin of eukaryotes crucial for understanding the evolution of complex life. It is a deeply mysterious event, but we do know it involved some sort of coming together of an archaean cell and a bacterium, in which the bacterium became the first mitochondrion.

The result was a massive energy boost for the host cell, but also a lot of new problems. The first eukaryotes essentially had two sets of genes 鈥� one archaeal, one bacterial 鈥� and they would have disrupted each other. The risk of lethal mutations must have been high.

Over the past 5 years,聽 and William Martin at the University of D眉sseldorf in Germany have suggested one way the first eukaryotes might have coped. The solution, they say, was for . 鈥淵ou鈥檙e talking about a host cell that is not dividing, but the nucleus continues to divide,鈥� says Garg.

Having multiple nuclei would have given the first eukaryotes a buffer. If one copy of a gene became damaged, another nucleus would still have a working version. As a result, the cell itself could experience the advantages of genetic mutation without the disadvantages. Not only would it be less likely a lethal genetic mutation would kill the eukaryote, but with all those nuclei, there would have been more chance of a given gene in one nucleus mutating into a form that significantly improved the ability of the eukaryote to survive in its environment.

鈥淚t permits more flexibility, more genetic diversity,鈥� agrees Javaux.

Now Garg has evidence. Multinucleate cells are widespread among eukaryotes: , and our bodies contain bone cells called that are multinucleate. Garg and his colleagues compiled data on 106 eukaryotic groups, noting which of them were known to make multinucleate cells.

Based on how the different groups are related, the researchers worked back to the shared ancestor of all of them. They concluded that the ability to form multinucleate cells probably dates back to the last shared ancestor of all modern eukaryotes.

That鈥檚 intriguing, says Javaux, but 鈥淚鈥檓 not sure it鈥檚 enough to prove it鈥�. A key question 鈥� one Garg acknowledges 鈥� is that we don鈥檛 know how most eukaryotic multinucleate cells arise. It can happen when a cell doesn鈥檛 divide 鈥� which is what Garg is proposing for the first eukaryotes 鈥� but it can also happen when two or more cells fuse. 鈥淚f you don鈥檛 know the origin of that state in each supergroup, it鈥檚 difficult to say if this is an ancestral state or a convergence of evolution,鈥� says Javaux.