Two nice papers on evolution have appeared this week.
Eukaryotes may be way older than we think. A paper in this week’s Nature (with Roger Buick’s News and Views summary here) reports the finding of eukaryote-like microfossils in South American rocks dated at 3.2 billion years b.p. (before the present). Yes, that’s right: 3.2 billion. Up to now, the oldest known fossils of eukaryotic cells (i.e., “true” cells with chromosomes and a nucleus) dated 1.8 billion years b.p., so it was assumed that it took a long time to evolve the eukaryotic cell from the bacterial cell (bacterial microfossils first appear about 3.5 billion years ago). This new finding, if true, pushes back the origin of true cells very close to the origin of prokaryotic cells themselves, so it may not have been that difficult to evolve a true cell.
The “acritarchs” found by Javaux et al. may be eukaryotes, but there’s some doubt. They are large (up to 0.3 mm, which might be visible to the naked eye), but some bacteria are also that large. And while they have “organic walls” around them (see Fig. 1), these are nowhere near as complex as the cells walls of eukaryotic algae or membranes of eukaryotic protists, complexity seen in much younger true eukaryotic fossils. On the other side, as reported by Roger Buick in his News & Views on this paper, sterane “biomarker” molecules (organic molecules that are uniquely produced by eukaryotes) have been found in deposits that are 2.45 billion years old, suggesting that maybe Javaux et al. are right.
We’ll have to wait and see about this, but the Javaux et al. paper suggests that we may have to radically rethink our views about the origin of cells. They might have appeared 1.4 billion years earlier than we thought! In the meantime, I’m revising my lecture notes.
Fig. 1. Some of the microfossils from the Javaux et al. paper (Figure taken from the News & Views). a.b., acritarchs. c,d. TEM images of wall ultrastructure of these creatures: a simple monolatered wall; e.,f: cell walls from 1.4 bya eukaryotic fossils, showing complex and multilayered organization.
Sick ants commit altruistic suicide. A paper by Jürgen Heinze and Bartosz Walter, in the latest issue of Current Biology, is not so earth-shaking but still really cute. (See the Dispatch from Michel Chapuisat.) The title tells the whole story: “Moribund ants leave their nests to die in social isolation.” (Chapuisat gives the better, nonacademic translation of it: “Sick ants face death alone.” Doesn’t that tug your heartstrings?)
Heinze and Walter studied the ant Temnothorax unifasciatus from Slovenia (confusingly, AntWeb describes it as being from Madeira and the Azores, so I don’t know for sure), observing that most individuals died when infected by the pathogenic fungus Metarhizium anisopliae. And most of these doomed ants leave the nests and die outside, well away from their nestmates and the underground brood.
Could this be an adaptive response by the ants to keep them from infecting their nestmates and causing a huge pandemic? Possibly, but there’s another explanation: some fungi that infect ants have evolved to manipulate the ant’s behavior to better spread the fungus. (That’s another great story that I’ll relate some day.) In other words, the behavior evolved to help the fungus reproduce; presumably fungi spread better when their carriers die in the open than within the colony.
Heinze and Walter controlled for this possibility by gassing the ants with carbon dioxide, which doesn’t kill them outright but shortens their lifespan. More than 80% of the ants who died left the nest before their deaths (an average of 36 hours before). The authors conclude that nest-leaving is characteristic of dying ants and not just a result of the fungus manipulating its host. “Nest leaving” may thus be a generally adaptive behavior to avoid infecting the colony.
How did this evolve? If it is indeed an adaptation, the likely process involved kin selection, which is of course responsible for the evolution of every “altruistic” behavior in worker ants, who are sterile. Presumably those genes promoting nest-abandonment-when-ill would be favored because the copies of those genes in the dying ant’s reproducing kin (males and the queen) would have a better chance of surviving (and founding future nests).
All this reminds me of the heroic and altruistic tent-leaving behavior of Captain Oates during the Scott polar expedition of 1912.
Javaus, E. J., C. P. Marshall, and A. Bekker. 2010. Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits. Nature 463: 934-938.
Heinze, J. and B. Walter. 2010. Moribund ants leave their nests to die in social isolation. Current Biology 20:249-252.