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.
I thought the ant mind-control fungus was already featured? (or maybe I read it somewhere else)
Did the authors look at sensory damage which may cause ants to leave the nest? After all we try to encourage rats to eat cholinesterase inhibitors because a convenient side effect is that the rats seek better lit places (so are less likely to die somewhere difficult to get at and thus stink up the place for a long time).
Bacteria and archaea both have cell walls (with a few exceptions).
Animal eukaryotes do not.
Plant and fungal eukaryotes do have walls, but they are different from each other and from those of bacteria and archaea (i.e. evolved independently later).
So the presence of cell walls in the fossils is not evidence in favor of them being eukaryotes.
Thanks. Corrected. What I mean is that the walls aren’t nearly as complex as the surrounding structures seen in later eukaryotic organisms.
I believe the first signs of life in the fossil record are 3.8 billion years old. 600 million years is an awful lot of time. It would not be too surprising having eukaryotic live at 3.2 billion years ago. Keeping in mind the fragmentary record of fossilization.
This is a pretty big rewrite of eukaryotic origins, isn’t it, if this is confirmed? It seems like pretty big news to me.
600 years IS an awful lot of time, but it also depends on the physical conditions at that time — it may well be that the necessary innovations for the eventual evolution of eukaryotes could not have formed under the climatic conditions of the first 600 years or so. (eg. certain proteins, membrane structures, genome architecture, etc…)
It is a fancy trend to only consider each of the elements (climate, geology, cell biology, genetics, biochemistry, etc) in isolation of each other, and they can yield drastically different and incoherent stories. Again, the sole author I have come across who actually considers data from a broad spectrum of fields, at times speculating a bit too wildly perhaps (but, unlike crappy crazies, undogmatically and still very well-argued and substantiated — his crazy is made of quality!) would be Tom Cavalier-Smith (see TC-S 2006 Biol Direct: “Rooting the tree of life by transition analyses” (open access) as well as TC-S 2006 Phil Trans R Soc B (free access).
I’m 100% sure I’ve read about the nest-leaving dying ants in some popular science book. I don’t remember which one, but I’ve definitely heard that before.
Well, this is confusing.
But let me get the OP off my chest first:
Is this a No True Cell fallacy, or is there an eukaryote independent definition here?
So to my confusion.
First, I note that Javaux is continuing a trend of his. I can cite a critic of his (with an agenda, as his theory predict eukaryotes as recent):
“Identity of the largest, most complex fossils ca 1.5 Gyr ago often called eukaryotic (Javaux et al. 2001, 2003) is problematic. While some might be stem eukaryotes (Javaux et al. 2003), this is not compellingly demonstrated. I consider it more likely that most, probably all, are simply unusually large and complex prokaryotes. Unlike Javaux et al. (2001), I do not think their morphology demands the presence of an internal cytoskeleton or endomembranes; it is within the morphogenetic capabilities of bacteria. […] [Cut to length out of context; Cavalier-Smith, “Cell evolution and Earth history: stasis and revolution“.]
Is it really testing a putative observation by accumulating more of these artifacts that are “within the morphogenetic capabilities of bacteria”?
Second, that thing about steranes. Just before that in the very same paper:
“The marked discrepancy between these late morphological records of all fossils unambiguously attributable to specific eukaryotic phyla and the very early occurrence of steranes often interpreted as eukaryotic derivatives (Brocks et al. 1999) is explained if the 2.7 Gyr ago steranes actually came from eubacteria, at least four groups of which can make sterols (Cavalier-Smith 2002a; Pearson et al. 2003). Of these, Mycobacteria make cholesterol like eukaryotes, and belong to actinobacteria, the probable ancestors of eukaryotes. Sterol biosynthesis evolved polyphyletically by modifying universal eubacterial isoprenoid metabolism after atmospheric oxygenation made it mechanistically possible, long before eukaryotes. It was probably vertically inherited by the first eukaryote from an actinobacterium. Claims that mycobacterial sterol synthesis enzymes were laterally transferred from eukaryote DNA have not been substantiated by phylogenetic analysis (Cavalier-Smith & Chao 2003b), and reflect the false assumption that the ancestors of eukaryotes were archaebacteria, not modified derivatives of actinobacteria that generated the eukaryote/archaebacterial cenancestor. Fossil steranes are not sound evidence for eukaryotes (Cavalier-Smith 2002b); even were they specific for eukaryotes, the potential for downward mobility of petroleum fractions containing them is a perpetual worry for age authenticity—this problem is absent for body fossils, though their identification as eukaryotic or bacterial is also sometimes practically impossible, sometimes overoptimistic.”
Look, I’m just a layman with an interest in abiogenesis. (From early on, but spurred by the freaking creationists!) These are complicated matters which I’m fumbling about in, and any and all help would be appreciated.
For example, AFAIU lateral gene transfers are minute in practice, IIRC I’ve seen numbers of less than 1 % of genes mentioned. It is also unparsimonious. So why wouldn’t one try a conventional phylogenetic tree first and foremost?
Moreover in this case, wouldn’t extra-ordinary claims need extra-ordinary evidence instead of the weaker evidence of LGT?
If four (4!) groups of eubacteria can make sterols, are steranes really biomarkers for eukaryotes? Steranes which according to Wikipedia “are a class of 4-cyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation.”
As I said, confusing.
Oops. In case it isn’t obvious, I’m cut off from the Javaux article by a pay-wall. I will get access and read it later, this is interesting stuff!
Oops again. I presumed; it is a trend of hers. My apologies!
Dammit, I specifically tolerated my painfully slow internet to load this page JUST so I could post scathing passages from TC-S 2006 Phil Trans R Soc B. And then you beat me to it. *cries*
I’d trust genetic, biochemical and cytological data far more than microfossils, as not only is it hard enough to distinguish prokaryotic from eukaryotic fossils at that stage, it’s even difficult to distinguish biotic from abiotic patterns, as the latter can have rather elaborate arrangements due to the wonderful self-organising properties of some chemicals. Furthermore, to support an earlier origin of Eukaryotes it would be seriously helpful to have evidence supporting an earlier origin of their sister clade, the Archaebacteria. Thus far, no such convincing evidence exists (Cavalier-Smith 2006 Biol Direct, Cavalier-Smith 2010 Phil Trans R Soc B, etc)
Furthermore, evidence explained in painful detail in the aforementioned TC-S 2006 PTRSB paper suggests that most extant eukaryotic radiation occured quite recently, after the last snowball earth event 600-700 million yrs ago. IIRC, this happens slightly before the Cambrian Explosion, which isn’t too surprising, although the latter was likely caused by an “anal breakthrough” (TC-S 2006 PTRSB, direct quote) wherein the subsequent polarisation of anal and oral regions enabled greater cephalisation, access to a greater design space, etc.
Anyway, tl;dr — count me in among those skeptical of an earlier origin of eukaryotes. I’d cite someone in addition to Cavalier-Smith, but he seems to be -about- the only person who dares to actually study bacterial evolution in an interesting and meaningful way — that is, by actually examining the organisms themselves rather than blindly running sequence trees one after another, screaming “Lateral gene transfer! Bacterial evolution is impossible to decipher!” at every opportunity.
Thank you Jerry, Torbjörn, and Psi Function! Wonderfully informative science writing from all of you.
Whoa, hold on guys, our vast Darwinist conspiracy is having a weak moment. Remember! Circular Dating!!!! These South American rocks are obviously NOT 3.2 billion years old! These South American rocks are obviously 1.8 billion years ago – see the fossils! We must NOT let evidence damage our theory! Now, quick, damage control – show no cracks!!
How is “eukaryote” being defined here? There are both morphological and biochemical differences between eukaryotes and prokaryotes. Are we talking about an organism with the many membrane-bound organelles like we see in modern eukaryotes, or are we talking about a primitive organism with a single membrane but that later multi-membraned organisms evolved from?
I think they are pretty clear that they don’t see “many membrane bound organelles”, but I want to know if they had a membrane bound nucleus.
It looks like they did… and that seems like as good a definition as any for the first eukaryotes.
This is exciting, as it suggests a greater likelihood for finding complex life elsewhere in our universe. After all, it only took 1 or 2 billion years on our planet.
It is pretty clear what a eukaryote is (at the moment), as they very neatly form a solid clade. Thus far, all eukaryotes have membrane-bound nuclei, cytoskeleton, endomembrane trafficking (eg. golgi and golgi-like structures, phagocytosis), mitochondria and their derivatives (incl. hydrogenosomes and mitosomes), as well as a wide array of genetic and biochemical synapomorphies.
To complicate things a bit, however (3.5 billion years is quite sufficient for evolution to spit out a giant mess, as you can see), many bacteria also have a cytoskeleton, internal membranes (eg. planctomycetes) as well as potential for membrane traffic of some sort (the TEM of the cytoplasm near the surface of Epulopiscium is quite…weird — full of strange invaginations, etc, of unknown function. Forgot the citation, sorry…and lack subscription access at the moment)…speaking of Epulopiscium (up to 800um long), large-ish cell size is not unique to eukaryotes! (stomatal guard cells in plants are about 20-30um, for comparison). Lastly, prokaryotes can be truly multicellular, with spatial differentiation and all that — eg. Streptomyces. Thus, if some extant or extinct prokaryote can form a giant “eukaryote-looking” cell, I wouldn’t be shocked in any way.
To definitively prove something as a eukaryote from a fossil sample…would be VERY difficult. Prokaryotes are far more biochemically/metabolically diverse and able than eukaryotes could ever dream to be, so biochemistry is risky in this case. Internal structures could well be artefacts of preservation or just intricate mineral growth. Also, what if this is some symbiotic association of several bacterial species, each forming different structures? There’s a documented case of bacterial symbiosis which yields an interesting organism complex… again, forgot the citation/name… (being away from home kinda sucks sometimes.)
Basically, this “primitive” life is way cooler than our wildest imaginations, so “it looks complex” is a VERY poor excuse to associate something closer to ourselves. That’s the point of my spamming this poor thread with various crap about prokaryotic/early eukaryotic evolution — the “primitive –> complex” life dichotomy is a crude and unproductive way of looking at things, and yet so often implied even in professional literature…
Ok I’m done spamming >_>
“The “acritarchs” found by Javaux et al. may be eukaryotes, but there’s some doubt.”
Aha!
The evilutionists have doubts! They can’t agree on anything!
“They might have appeared 1.4 billion years earlier than we thought! In the meantime, I’m revising my lecture notes.”
And that is the difference between science and religion.
I work with fossil dinoflagellates. These are mainly marine phytoplankton with a benthic resting stage. It is the cell wall of the resting spore, the hypnozygote, or “cyst”, that is preserved in sediments. The cyst wall is composed of a macromolecule of complex polysaccharides. Living dinoflagellates construct this extremely strong, acid-resistant material by polymerising their photosynthesising pigments during their motile stage. An economic strategy, they take what was expensive to make (photosynthesising pigments) and use it to make a different material that they need for their survival.
The preparation method we use to extract dinoflagellate cysts from Mesozoic and Cenozoic sedimentary rock is the same as the method Javaux, Marshall & Bekker used to extract organic-walled microfossils from 3.2 billion year South African (not South American) rocks. First, we dissolve the carbonate minerals in the sediment in warm hydrochloric acid. Then we dissolve the silicate fraction of the residue in warm 60% hydrofluoric acid. What remains after this brutal treatment is acid-resistant organic material: marine phytoplankton resting spores, acritarchs (unknown affinities), pollen and spores from land plants (the latter only in rocks younger than the evolution of land plants, of course).
It is a testament to the impressive chemical resistance of the complex macromolecules that such microfossils are found in 3.2 million year old sediments. If these are in-situ specimens, this is a very important find for palaeontology. Eukaryotes or prokaryotes, it is a wonderful discovery.
[…]that such microfossils are found in 3.2 million year old sediments.
Ooops, sorry. 3.2 BILLION it should say. The spell checker didn’t catch it 😉
On altruistic ants articles see my blog at http://www.vivanewmexico.com/blog/wp-admin/post.php?action=edit&post=1375
Amazing how the stories get modified.