needed for oxygen respiration. Everything else has gone. So the paradox is this: if the eukaryotic cell was supposedly born of a symbiosis between an oxygen-hating methanogen and an oxygen-loving bacterium, how could the methanogen possibly benefit from having α-proteobacteria inside it? For that matter, how did the α-proteobacteria benefit from being inside? Indeed, if the host was incapable of phagocytosis—and methanogens are certainly not able to change shape and eat other cells—how on earth did it get inside?
It is possible that Siv Andersson’s Ox-Tox hypothesis still applies—in other words, the oxygen-guzzling bacterium protected its host from toxic oxygen, enabling the methanogen to venture into pastures new. But there is a big difficulty with this scenario now. Such a relationship makes sense for a primitive archezoon that lives by fermenting organic remains. This will prosper if it is able to migrate to any environment where such remains can be found. Such scavenging cells are the single-celled equivalent of jackals prowling Africa, covering vast distances in the search for a fresh carcass. But this roving existence would kill a methanogen. A methanogen is as tied to a low-oxygen environment as a hippo is to waterholes. The methanogens can
tolerate
the presence of oxygen, but they can’t generate any energy in its presence, becausethey depend on hydrogen for fuel, and this is very rarely found in the same environment as oxygen. So if a methanogen does leave its watering hole, it must starve until it gets back: festering organic remains mean nothing to a methanogen—it would do better never to leave. Thus there is a deep tension between the interests of the methanogen, which gains nothing from venturing to pastures new, and those of an oxygen-guzzling parasite, which can’t generate any energy at all in the anoxic environment favoured by methanogens.
This paradox is heightened because, as we have seen, their relationship could not have depended on energy in the form of exchangeable ATP—bacteria do not have ATP exporters, and never benevolently ‘feed’ each other. The tryst could still have been a parasitic relationship, in which the bacteria consumed the organic products of the methanogen from within—but again, there are problems with this, as an oxygen-dependent bacterium could not generate any energy from the innards of a methanogen unless it could persuade the methanogen to leave its waterhole, those comfortable oxygen-free surroundings. One might picture the α-proteobacteria herding the methanogens and driving them like cattle to an oxygen-rich slaughter field, but for bacteria this is nonsense. In short, the methanogens would starve if they left their waterhole; the oxygen-dependent bacteria would starve if they lived in the waterhole, and the middle ground, a little oxygen, must have been equally disadvantageous to both parties. Such a relationship seems to be mutually insufferable—is this really how the stable symbiotic relationship of the eukaryotic cell began? It is not just improbable, but downright preposterous. Luckily there is another possibility, which until recently seemed fanciful, but is now looking far more persuasive.
3
The Hydrogen Hypothesis
The quest to find the progenitor of the eukaryotic cell has run into dire straits. The idea that there might have been a primitive intermediate, a missing link with a nucleus but no mitochondria, has not been rigorously disproved, but looks more and more unlikely. Every promising example has turned out
not
to be a missing link at all, but rather to have adapted to a simpler lifestyle at a later date. The ancestors of all these apparently primitive groups
did
possess mitochondria, and their descendents eventually lost them while adapting to new niches, often as parasites. It seems possible to
be
a eukaryote without having mitochondria—there are a thousand such species among the protozoa—but it does not seem possible to be a
It is possible that Siv Andersson’s Ox-Tox hypothesis still applies—in other words, the oxygen-guzzling bacterium protected its host from toxic oxygen, enabling the methanogen to venture into pastures new. But there is a big difficulty with this scenario now. Such a relationship makes sense for a primitive archezoon that lives by fermenting organic remains. This will prosper if it is able to migrate to any environment where such remains can be found. Such scavenging cells are the single-celled equivalent of jackals prowling Africa, covering vast distances in the search for a fresh carcass. But this roving existence would kill a methanogen. A methanogen is as tied to a low-oxygen environment as a hippo is to waterholes. The methanogens can
tolerate
the presence of oxygen, but they can’t generate any energy in its presence, becausethey depend on hydrogen for fuel, and this is very rarely found in the same environment as oxygen. So if a methanogen does leave its watering hole, it must starve until it gets back: festering organic remains mean nothing to a methanogen—it would do better never to leave. Thus there is a deep tension between the interests of the methanogen, which gains nothing from venturing to pastures new, and those of an oxygen-guzzling parasite, which can’t generate any energy at all in the anoxic environment favoured by methanogens.
This paradox is heightened because, as we have seen, their relationship could not have depended on energy in the form of exchangeable ATP—bacteria do not have ATP exporters, and never benevolently ‘feed’ each other. The tryst could still have been a parasitic relationship, in which the bacteria consumed the organic products of the methanogen from within—but again, there are problems with this, as an oxygen-dependent bacterium could not generate any energy from the innards of a methanogen unless it could persuade the methanogen to leave its waterhole, those comfortable oxygen-free surroundings. One might picture the α-proteobacteria herding the methanogens and driving them like cattle to an oxygen-rich slaughter field, but for bacteria this is nonsense. In short, the methanogens would starve if they left their waterhole; the oxygen-dependent bacteria would starve if they lived in the waterhole, and the middle ground, a little oxygen, must have been equally disadvantageous to both parties. Such a relationship seems to be mutually insufferable—is this really how the stable symbiotic relationship of the eukaryotic cell began? It is not just improbable, but downright preposterous. Luckily there is another possibility, which until recently seemed fanciful, but is now looking far more persuasive.
3
The Hydrogen Hypothesis
The quest to find the progenitor of the eukaryotic cell has run into dire straits. The idea that there might have been a primitive intermediate, a missing link with a nucleus but no mitochondria, has not been rigorously disproved, but looks more and more unlikely. Every promising example has turned out
not
to be a missing link at all, but rather to have adapted to a simpler lifestyle at a later date. The ancestors of all these apparently primitive groups
did
possess mitochondria, and their descendents eventually lost them while adapting to new niches, often as parasites. It seems possible to
be
a eukaryote without having mitochondria—there are a thousand such species among the protozoa—but it does not seem possible to be a
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