Life Without Sunlight: The Deep-Sea Vent Communities

AThe discovery in 1977 of dense animal communities clustered around hot springs on the Galapagos Rift was received as proof that life could be built without sunlight, and it was also, almost at once, received as proof of something rather different. Because the chemistry feeding those communities demands nothing that was unavailable to the earliest oceans, the animals themselves were widely assumed to be extremely old: survivors that had ridden out the asteroid impacts, the anoxic episodes and the glaciations which repeatedly emptied the sunlit water above them. The phrase living fossil appeared in three of the first four descriptive papers and has proved unusually hard to dislodge. It carried a further implication that few of those who used it stopped to examine, namely that a habitat independent of the surface must also be insulated from whatever happens there. Nobody troubled to test the idea, because nothing about it seemed to require testing. Two decades of dating work have made that implication difficult to sustain, and the argument which followed is less about the vents themselves than about what an assemblage of animals gathered in one place may fairly be taken to prove about its own past.

BTwo independent lines of evidence point the same way. The first is genetic: molecular clocks, calibrated against events whose timing is known from other rocks, place the separation of the principal vent lineages of tube worms, mussels and limpets within the past 60 million years, a small fraction of the age of the ridge system on which they sit. The second is palaeontological. Fossil vent deposits are rare, yet around one hundred have been described since 1985, and they do not contain the animals now living at vents. Jurassic chimneys some 160 million years old are dominated by brachiopods with no representatives at any modern vent, while the great clams and worms of the present communities turn up only in deposits younger than about 50 million years. The occupants, in short, have been replaced, and replaced more than once, although the habitat itself has been continuously available throughout. The pattern is not a subtle one, and it does not rest on any single deposit.

CAnneke Vos, of the Delfzijl Institute of Marine Palaeontology, has drawn the sharpest conclusion from that pattern. She argues that a vent is not a cradle in which lineages arise but an evolutionary sink into which they repeatedly fall, since animals already adapted to sulphide arrive from other chemical habitats, flourish for a period measured in tens of millions of years, and then vanish, to be succeeded by newcomers drawn from the same reservoirs. Between 2009 and 2014 her group re-examined 214 fossil genera from 46 deposits and found that only nine of them spanned more than two of the intervals into which the record is conventionally divided. Her reservoirs are the cold seeps of the continental slopes and the sunken carcasses of the open ocean, habitats that are chemically comparable and a great deal more widely scattered. She is careful to bound the claim. The turnover is plain only among the larger-bodied groups, whose shells survive well enough to be counted, and she concedes that a lineage of small soft-bodied worms might persist for an immense time while leaving nothing whatever for anyone to find.

DEmil Sagredo, of the Ligurian Marine Station, contends that the fossil argument is close to worthless, because the rock that ought to carry the evidence destroys itself. A sulphide chimney turns chemically unstable the moment its fluid supply is cut off, and unlike the carbonate mounds that form at cold seeps it seldom lasts long enough to be buried intact, so that the hundred known deposits are, on his estimate, a thousandth of what once existed. Absence from a record of that kind, he insists, is absence of rock rather than absence of animals. The objection is well taken as far as it goes, and Vos accepts the arithmetic without protest. It nevertheless leaves the genetic evidence entirely untouched, a point Sagredo himself concedes, since a molecular clock is read from the tissue of living animals and needs no fossil at all. His own reconstruction, moreover, rests on preservation models tested against just three ancient deposits, every one of them taken from a single margin. One margin, he agrees, is not a world.

EThe dispute looks a good deal less severe once two questions are pulled apart, which is what neither side troubled to do for many years. How old is the manner of living, and how old are the animals now practising it, are separate matters, and the evidence answers them differently. Chemosynthesis is almost certainly ancient, older than any lineage now surviving and perhaps older than photosynthesis; the tube worms are not. Between those two statements sits the mechanism that keeps the habitat occupied, and here the quarrel has been quietly productive. The carcasses of large whales, known as whale falls, sink to the sea floor and support a sulphur-based community for decades, and they are now widely regarded as the stepping stones by which colonists cross the gap from one field to the next, whereas cold seeps, chemically similar but lying for the most part on continental margins, look a less likely route. What follows is a change in the kind of question worth putting to a vent field. Vents are old addresses with new tenants. That is a duller sentence than the one written in 1977, and it is much better supported.