Read more: 鈥�Not so simple: Bugs that break all the rules鈥�
One bacterium breaks two rules: it鈥檚 got something like a nucleus, and it swallows its food
The defining characteristic of simple cells, or prokaryotes, is supposed to be the fact that their DNA floats freely in the cell rather than being enclosed in a nucleus. Indeed, the term 鈥減rokaryote鈥� means 鈥渂efore nucleus鈥�.
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Then, in 1991, John Fuerst and Richard Webb of the University of Queensland in Brisbane, Australia, noticed something strange about Gemmata obscuriglobus, a bacterium first described in 1984. It seemed to have 鈥減ackaged鈥� DNA. Images taken with an electron microscope confirmed that not only was its DNA separate from the rest of the cell, it was enclosed by a double membrane 鈥� just like the membrane envelope surrounding the nucleus of complex cells.
So is G. obscuriglobus, along with other members of the Planctomycete group to which it belongs, really a prokaryote? Its nucleus, such as it is, is not nearly as sophisticated as that of complex cells, and genetically it is very much like other bacteria. So many biologists were inclined to treat its 鈥渘ucleus鈥� as a curiosity unrelated to that of complex cells 鈥� until this bacterium was found to have another extraordinary trait鈥�
They can swallow
Complex cells like amoebae (pictured right) and white blood cells can 鈥渟wallow鈥� large particles by engulfing them and pinching off the 鈥渂ubble鈥� containing the particle, a process called endocytosis. Bacteria were only thought to be able to take in particles via channels in their cell membrane, through which only small particles can fit. But last year, a team led by Fuerst showed that the bacterium with a 鈥渘ucleus鈥�, G. obscuriglobus, swallows large particles in a manner akin to endocytosis ().
To have one trait possessed by complex cells 鈥� membrane-bound DNA 鈥� could be a coincidence. To have two seems unlikely. What鈥檚 more, most members of the larger group that G. obscuriglobus belongs to, the Planctomycetes-Verrucomicrobia-Chlamydiae or PVC group, have proteins that are very similar to those that control endocytosis in eukaryotes. The big question is whether this is a case of parallel evolution, or whether complex cells and bacteria shared a common ancestor capable of endocytosis.
If there was a common ancestor, the implications are huge. It means the shared ancestor 鈥� known as the last universal common ancestor, or LUCA 鈥� and its contemporaries must have been much more complex than they have always been assumed to be. Since the ability of PVC bacteria to form membranes around their nucleus involves many of the same proteins as endocytosis, it is even possible that LUCA had a membrane-bound nucleus too. This would . Rather than being 鈥減rimitive鈥� cells, modern bacteria may be streamlined, simplified versions of a more complex ancestor 鈥� perhaps not so much prokaryotes as 鈥減ost-karyotes鈥�.
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Read more: 鈥�Not so simple: Bugs that break all the rules鈥�
