eukaryote without once having
had
mitochondria, deep in the past. If the only way to be a eukaryotic cell is via the possession of mitochondria, then it might be that the eukaryotic cell itself was originally crafted from a symbiosis between the bacterial ancestors of the mitochondria and their host cells.
If the eukaryotic cell was born of a merger between two types of cell, the question becomes more pressing—what types of cell? According to the textbook view, the host cell was a primitive eukaryotic cell, without mitochondria, but this obviously can’t be true if there never was a primitive eukaryotic cell that lacked mitochondria. In her endosymbiosis theory, Lynn Margulis had in fact proposed a union between two different types of bacteria, and her hypothesis looked set for a return to prominence after the demise of the missing link. Even so, Margulis and everyone else were thinking along the same lines—the host, they imagined, must have relied on fermentation to produce its energy, in the same way that yeasts do today, and the advantage that the mitochondria brought with them was an ability to deal with oxygen, giving their hosts a more efficient way of generating energy. The exact identity of the host could potentially be traced by comparing the gene sequences of modern eukaryotes with various groups of bacteria and archaea—and modern sequencing technology was just beginning to make that possible. But, as we have just seen, the apparent answer came as another shock: the genes of eukaryotic cells seem to be related most closely to
methanogens
, those obscure methane-producing archaea that live in swamps and intestines.
Methanogens! This answer is an enigma. In Chapter 1 , we noted that the methanogens live by reacting hydrogen gas with carbon dioxide, and evanescing methane gas as a waste product. Free hydrogen gas only exists in the absence of oxygen, so the methanogens are restricted to
anoxic
environments—any marginal places where oxygen is excluded. It’s actually worse than that. Methanogens can tolerate some oxygen in their surroundings, just as we can survive underwater for a short time by holding our breath. The trouble is that methanogens can’t generate any energy in these circumstances—they have to ‘hold their breath’ until they get back to their preferred anoxic surroundings, because the processes by which they generate their energy can
only
work in the strict absence of oxygen. So if the host cell really was a methanogen, this raises a serious question about the nature of the symbiosis—why on earth would a methanogen form a relationship with any kind of bacteria that relied upon oxygen to live? Today, modern mitochondria certainly depend on oxygen, and if it was ever thus, neither party could make a living in the land of the other. This is a serious paradox and did not seem possible to reconcile in conventional terms.
Then in 1998, Bill Martin, whom we met in Chapter 1 , stepped into the frame, presenting a radical hypothesis in
Nature
with his long-term collaborator Miklós Müller, from the Rockefeller University in New York. They called their theory the ‘hydrogen hypothesis’, and as the name implies it has little to do with oxygen and much to do with hydrogen. The key, said Martin and Müller, is that hydrogen gas can be generated as a waste product by some strange mitochondria-like organelles called
hydrogenosomes
. These are found mostly among primitive single-celled eukaryotes, including parasites such as
Trichomonas vaginalis
, one of the discredited ‘archezoa’. Like mitochondria, hydrogenosomes are responsible for energy generation, but they do this in bizarre fashion by releasing hydrogen gas into their surroundings.
For a long time the evolutionary origin of hydrogenosomes was shrouded in mystery, but a number of structural similarities prompted Müller and others, notably Martin Embley and colleagues at the Natural History Museum in London, to propose that
had
mitochondria, deep in the past. If the only way to be a eukaryotic cell is via the possession of mitochondria, then it might be that the eukaryotic cell itself was originally crafted from a symbiosis between the bacterial ancestors of the mitochondria and their host cells.
If the eukaryotic cell was born of a merger between two types of cell, the question becomes more pressing—what types of cell? According to the textbook view, the host cell was a primitive eukaryotic cell, without mitochondria, but this obviously can’t be true if there never was a primitive eukaryotic cell that lacked mitochondria. In her endosymbiosis theory, Lynn Margulis had in fact proposed a union between two different types of bacteria, and her hypothesis looked set for a return to prominence after the demise of the missing link. Even so, Margulis and everyone else were thinking along the same lines—the host, they imagined, must have relied on fermentation to produce its energy, in the same way that yeasts do today, and the advantage that the mitochondria brought with them was an ability to deal with oxygen, giving their hosts a more efficient way of generating energy. The exact identity of the host could potentially be traced by comparing the gene sequences of modern eukaryotes with various groups of bacteria and archaea—and modern sequencing technology was just beginning to make that possible. But, as we have just seen, the apparent answer came as another shock: the genes of eukaryotic cells seem to be related most closely to
methanogens
, those obscure methane-producing archaea that live in swamps and intestines.
Methanogens! This answer is an enigma. In Chapter 1 , we noted that the methanogens live by reacting hydrogen gas with carbon dioxide, and evanescing methane gas as a waste product. Free hydrogen gas only exists in the absence of oxygen, so the methanogens are restricted to
anoxic
environments—any marginal places where oxygen is excluded. It’s actually worse than that. Methanogens can tolerate some oxygen in their surroundings, just as we can survive underwater for a short time by holding our breath. The trouble is that methanogens can’t generate any energy in these circumstances—they have to ‘hold their breath’ until they get back to their preferred anoxic surroundings, because the processes by which they generate their energy can
only
work in the strict absence of oxygen. So if the host cell really was a methanogen, this raises a serious question about the nature of the symbiosis—why on earth would a methanogen form a relationship with any kind of bacteria that relied upon oxygen to live? Today, modern mitochondria certainly depend on oxygen, and if it was ever thus, neither party could make a living in the land of the other. This is a serious paradox and did not seem possible to reconcile in conventional terms.
Then in 1998, Bill Martin, whom we met in Chapter 1 , stepped into the frame, presenting a radical hypothesis in
Nature
with his long-term collaborator Miklós Müller, from the Rockefeller University in New York. They called their theory the ‘hydrogen hypothesis’, and as the name implies it has little to do with oxygen and much to do with hydrogen. The key, said Martin and Müller, is that hydrogen gas can be generated as a waste product by some strange mitochondria-like organelles called
hydrogenosomes
. These are found mostly among primitive single-celled eukaryotes, including parasites such as
Trichomonas vaginalis
, one of the discredited ‘archezoa’. Like mitochondria, hydrogenosomes are responsible for energy generation, but they do this in bizarre fashion by releasing hydrogen gas into their surroundings.
For a long time the evolutionary origin of hydrogenosomes was shrouded in mystery, but a number of structural similarities prompted Müller and others, notably Martin Embley and colleagues at the Natural History Museum in London, to propose that
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