IT IS EVENING. In the depths of the ocean, off the coast of Africa, something
is stirring. From a cave, 200 metres below the surface, a group of about ten
fish emerge. It鈥檚 murky, but if you could see them clearly they would be cobalt
blue, speckled white and about the length of a person. Slowly, steadily they
swim away, each heading off to the deeper ocean for a night of solitary hunting.
Such a scene is not unusual. Something similar has been happening every evening
for the past 400 million years. For these fish are coelacanths, and they are
among the planet鈥檚 supreme survivors.
Coelacanths were roaming the oceans when their close relatives became the
first vertebrates to venture onto land. In the early days, they shared their
watery home with trilobites and a few primitive molluscs. The coelacanth group
reached its peak around 200 million years ago. 鈥淭ypes of coelacanth were once
found in all kinds of environments,鈥� says Peter Forey of the Natural History
Museum in London. 鈥淭hey have been gradually declining since the Triassic.鈥� As
far as we know, two small populations remain, off the Comoros near Madagascar
and around Sulawesi in Indonesia.
These ancient fish are among a select group of species that have so
spectacularly out-survived their evolutionary peers that Darwin called them
living fossils. Gingko trees, for example, would have been a familiar sight to
dinosaurs living 125 million years ago. Crocodiles have remained the same for
140 million years. Horseshoe crabs are 200 million years old, a type of lamp
shell called Lingula has existed for 450 million years, and molluscs of the
genus Neopilina鈥攚hich are intermediate between a mollusc and a
worm鈥攈ave been around for an astonishing 500 million years. What is the
secret of their success? Have they stumbled upon some formula for survival, or
is their continuing existence on Earth just down to an unlikely succession of
lucky breaks?
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First, let鈥檚 put this feat in context. According to David Jablonski of the
University of Chicago, 99.9 per cent of all the species that have ever existed
are now extinct. 鈥淭he average multicellular species lasts between one and ten
million years,鈥� he says, 鈥渁nd we鈥檝e had 600 million years of turnover.鈥� Five
mass extinctions have each killed off between 76 and 96 per cent of all species.
And there have been huge changes in the biological and physical environments on
Earth.
Without doubt, chance has played a part in the survival of living fossils.
鈥淭here is a big lucky aspect to this,鈥� says Niles Eldredge of the American
Museum of Natural History in New York, 鈥渟o you have to take it on a case by case
basis.鈥� For instance, he suggests that the lamp shell Lingula, which lives on
the seafloor, survived the mass extinctions that killed off most of its
relatives because it can bury itself in the sand and stay out of trouble.
Serendipity may also be the key to the tuatara鈥檚 survival. This reptile, of
the genus Sphenodon, lives only in New Zealand and pre-dates many of
the dinosaur groups. 鈥淭wo hundred million years ago, Sphenodontia were almost
worldwide,鈥� says Charles Daugherty of Victoria University of Wellington. 鈥淭hen,
80 million years ago, New Zealand broke free from Gondwanaland, the main
landmass. By chance New Zealand didn鈥檛 have, or subsequently lost, mammals.鈥� So
the tuatara thrived in New Zealand in the absence of mammals, while dying out
everywhere else in the world in the face of competition from them.
But luck alone cannot explain the longevity of the tuatara and a host of
other living fossils. Evolutionary biologists are trying to identify the various
strategies that underpin the success of these creatures. 鈥淥ne way to survive
forever is to be abundant and live everywhere, like the cockroach,鈥� says
Jablonski. Cockroaches, which have been around for 250 million years, are
ecological generalists. In other words, they are highly adaptable and
opportunistic, so they can live in a wide variety of environments. 鈥淭here is
case after case where people have compared living fossils to their closest
living relatives and found the relatives to be more specialised ecologically,
and therefore more prone to extinction,鈥� says Eldredge.
Impala, for example, may not be an ancient relic, but they have not changed
at all for 7 million years. During that time, their close relatives the
wildebeests and hartebeests have produced 32 species, most of which are now
extinct. This is because impala are not fussy and can live in a wide variety of
African ecosystems. If the vegetation changes, impala eat something different,
while other antelope become extinct and new species arise.
But being unspecialised does not explain the survival of all living fossils.
The coelacanth is specialised for a deep-sea life and is found only in volcanic
caves. 鈥淭his fish has the lowest oxygen consumption among all living
vertebrates,鈥� says Hans Fricke of the University of Munich. 鈥淪o it has a low
food requirement and can survive in areas of very low prey density.鈥� In these
deep-sea caves, the coelacanth found a habitat that has hardly changed for
millions of years.
And it is not the only group that owes its survival to its stable habitat.
鈥淭ake the heat-loving archaebacteria,鈥� says Elisabeth Vrba, a palaeontologist at
Yale University. 鈥淭hey evolved 3.5 billion years ago, when life was just
beginning and the Earth was very hot. Today, they live in water at boiling point
in places like Yellowstone Park, and the essential features of their habitat are
still the same.鈥�
While some living fossils owe their existence to avoiding external change,
others may have survived in their primeval form by slowing down their response
to change. The tuatara, for example, is extremely long lived. 鈥淭uataras don鈥檛
reach reproductive maturity until the age of 15,鈥� says Daugherty, 鈥渁nd they
don鈥檛 stop growing until 30. Individuals probably live for a century or
尘辞谤别.鈥�
Nautiluses, the sole survivors of a group of shelled molluscs related to the
octopus that dominated marine life around 300 million years ago, are also
long-lived and reproduce slowly compared with their living relatives. Slow
reproduction means that natural selection is slowed and helps explain why some
living fossils haven鈥檛 changed over time. 鈥淚t is nothing to do with survival,鈥�
says James Valentine, a palaeontologist at the University of California at
Berkeley. 鈥淚t is something in their life history that鈥檚 put them in an
evolutionary straitjacket.鈥�
But there are living fossils that have very short lives and reproduce
extremely quickly. Think of cockroaches and archaebacteria. So is there
some other reason why these creatures have not evolved, while everything around
them has? Perhaps they are somehow constrained, prevented from evolving.
Some biologists marvel that there is any evolution at all, considering the
possible pitfalls of change. 鈥淭he idea is that organisms are so complex that it
is very hard to change one aspect without wrecking everything else,鈥� says Vrba.
But it is extremely difficult to show that this is why our supreme survivors
remain unchanged for millions of years.
The other thing that might stop evolution is evolution itself, leaving you
stuck in a rut. Peter Holland, at the University of Reading, believes that one
animal has been held in suspended animation for 550 million years because it
didn鈥檛 have enough genes to improve its design. He works on amphioxi, or
lancelets, which live in marine sediment and look a bit like small fish. In
fact, they are similar to the ancestor of all the vertebrates and have hardly
changed since the Cambrian explosion. According to Holland amphioxi are as
complex as they can possibly be, given the number of genes they have.
鈥淎t some stage close to the origin of vertebrates, there was a duplication of
genes,鈥� he explains. Amphioxi missed out and still have about 20 000 genes,
while all the later vertebrates have more like 70 000. 鈥淲hether the duplication
was a chance event, or whether something prevented the acquisition of extra
genes in Amphioxus, we don鈥檛 know,鈥� he adds.
Evolutionary constraint can鈥檛 explain the persistence of all the living
fossils, though, because so many of them come from groups that used to contain a
great diversity of form. Crocodiles, for example, have stayed the same for 140
million years, but they come from a group of aquatic predators that had a great
range of shapes, sizes and lifestyles in the past. There were crocodiles with
strange tall snouts or duckbills, and some even had hooves. Whatever has kept
our modern crocodiles the same, it is certainly not that they were unable to
evolve into other types, because they did.
All this leaves a rather complicated picture for anyone seeking the secret of
exceptional survival. Be general, or specialised. Live fast, or slow. Keep it
simple, or don鈥檛. Be in the right place at the right time. If all else fails,
try becoming a 鈥渟uperspecies鈥�, blessed with a physiology that can withstand
anything.
Horseshoe crab embryos, for example, are much more resistant to pollution
than other marine arthropods, while gingko trees thrive in cities because they
cope well with atmospheric pollution. As for humans, maybe we have what it takes
to become a living fossil. Or maybe not.
THE average lifetime of a species is somewhere between one and fifteen
million years, depending on who you ask. It is also different for different
types of organism (see Diagram).
Marine species tend to persist for longer than terrestrial species, for instance,
perhaps because the marine environment is less changeable. Mammals are the jet set,
with species being created and lost at a high rate.
Michael Novacek from the American Museum of Natural History, thinks he can
explain this finding. He points out that mammals have a short generation time
and high reproductive rate compared with, say, bivalve molluscs. 鈥淪mall mammals
show very rapid genetic change,鈥� he says. So they change into new species more
quickly.
So, how long can we expect us humans to last? Physicist J. Richard Gott of
Princeton University has predicted that Homo sapienswill survive
between 0.2 and 8.1 million years. His calculation was based on the time we have
already been around. Since most science works on a 95 per cent level of
confidence, he assumed that the current point in time is not in the first 2.5
per cent or the last 2.5 per cent of the total duration of the human species. So
if the 200 000 years since we evolved is somewhere between 2.5 per cent and 97.5
per cent of our total time on Earth, we should have at least 5100 years to go.
But then you know what they say about statistics.