I do love the fact that there is always something new to learn. And often, to pass on to my students. Like the ‘Lost City’ – a surreal landscape of ghostly white towers that’s formed around alkaline vents deep under the Atlantic Ocean. Now, I know about the ‘black smokers’ – fragile black towers belching superheated, chemical-rich waters into the icy ocean depths of the mid-Atlantic ridge, and the amazing biological communities associated with them. I talk about them in lectures, as (among other things) an example of ecosystems that don’t rely on inputs from the sun to power their producers. And colleagues of mine have the privilege of regularly diving to study these pockets of life. But alkaline vents?
I encountered the vents in Nick Lane’s new book Life Ascending: the ten great inventions of evolution. Lane introduces them in the context of his chapter on the origins of life. We’ve long known that life evolved in the water, & for a while now at least some scientists have been talking about the possibility that the black smokers were the actual venue for this event. More recent still is the proposal that the cooler, chemically different alkaline vents offered a place for life to evolve. As Lane says on his blog, The origin of life is one of biology’s biggest conundrums. How prebiotic chemistry gave rise to biochemistry, how the first cells formed, what kind of energy first powered metabolism and replication — all these questions are serious challenges. Remarkably, all are answered in broad brush stroke by the amazing properties of alkaline hydrothermal vents, which form naturally chemiosmotic, self-replicating mineral cells with catalytic walls. They concentrate organics, including nucleotides, in impressive quantities, making them the ideal hatcheries for life.
While black smokers form along zones of sea-floor spreading, alkaline vents lie some tens of kilometres back from these rifts in the planet’s crust. And the minerals expelled from alkaline vents precipitate from the water to form white filigree towers – shown below in an image from the the best-known field, the ‘Lost City’.
Why do some scientists think that these ghostly towers could have been incubators for the first living things on Earth? One factor is their age. Black smokers are relatively short-lived: they & their communities decline & die over perhaps a few hundred years as the rocks they are on move away from the crustal rift & the fresh magma that powers them. But the pale vent towers last much longer; in the case of the Lost City, around 40,000 years so far. This significantly increases the odds of life evolving on or in them. And internally, at a microscopic level the vent towers contain huge numbers of interconnecting compartments that trap & concentrate the organic molecules formed by the reaction of carbon dioxide with the hydrogen gas bubbling up from the rocks beneath the vents. This would make the formation of organic molecules, including polymers like RNA, much more likely than around the black smokers. It’s suggested that the most likely pathway for this to happen is by a reversed Krebs cycle, which rather than breaking organic molecules down into CO2 & hydrogen, builds them up. And significantly, while the reversed Krebs cycle is unusual, it’s fairly common in those bacteria that live in alkaline vents.
In other words, this is a plausible hypothesis & one that offers plenty of opportunity for modelling & testing. And I’ve learned something new
N.Lane (2009) Life Ascending: the ten great inventions of evolution. Norton.