We wouldn't be here if the first primates hadn't evolved some
special features – to discover why it happened we have to go out on a
limb
BRING me his head. That was the job
given to graduate student Jonathan Bloch 20 years ago. His supervisor,
Philip Gingerich, had collected some large fossil-rich limestone blocks
from the
Bighorn basin in Wyoming and brought them back to the museum at the University of Michigan.
"The rocks had bone in them, but what
exactly was a mystery. It was my task to reduce the rock using acid to
see what I could find," says Bloch. "When I asked Philip what I should
be looking for, he said something like, 'how about a skull?'" Bloch said
OK and set to work. At the time, he didn't realise how incredibly rare
it is to find mammal skulls from the time after the death of the
dinosaurs 66 million years ago, when the limestone had formed. "Within
the first few days of work out popped the skull. I thought, well that is
good, I found what I was supposed to. Philip was very surprised. He had
the experience to recognise how big of a deal it was," Bloch recalls.
The skull was not just that of any old
mammal, but of a much sought-after "missing link" in the primate fossil
record. Fierce debate still rages over its significance, but many see
it as a crucial piece of evidence in the story of how humans came to be –
one that suggests flowers played a key role in our evolution.
Take a look at your hands and you'll
see they have evolved for grasping things, with opposable digits and
flat nails instead of claws. We also have forward-facing eyes, and
bigger brains than most other mammals. We tend to think of these traits
as human, but almost all primates share them too. So what made the
ancestors of primates evolve them in the first place, paving the way for
our evolution?
We know roughly when it happened. The
first steps in primate evolution were probably taken around 60 million
years ago, when the ancestor of all primates – thought to be a small,
nocturnal creature – took to the trees. The big question is why its
descendants evolved in the way they did.
The explanation might seem obvious:
when you take to the trees, you need grasping hands for clinging to
branches and forward-facing eyes for judging distance. But in the 1970s,
anthropologist Matt Cartmill pointed out that it can't be that simple.
Many mammals have opted for a life in trees and thrived without ever
evolving these features. Squirrels have sideways-facing eyes and claws
instead of nails, for instance, but they're perfectly at home leaping
from branch to branch. So there must be more to our eyes and hands than
that.
The extra factor, Cartmill suggested,
was catching insects. He pointed out that in living tree dwellers,
grasping hands and feet are usually found in animals that forage on
young branches too thin for claws to get a grip. Forward-facing eyes,
meanwhile, are common in predators, such as cats and owls, that rely on
vision to catch their prey. In particular, he argued, the big overlap in
the fields of vision of primate eyes is best for judging short
distances – whether an insect is within arm's length rather than whether
a branch is within leaping range.
So the key traits of our early primate
ancestors evolved, Cartmill proposed, because they were hunting insects
on fine branches. "It's a logical argument," says Robert Sussman, who
studies primate evolution at Washington University in St Louis. But, he
adds, it depends largely on comparisons with living animals rather than
the fossil record.
Fossil teeth suggest that insects were
not the only food of early primates. Their flat, round molars were
better suited to grinding fruit and plant material than they were to
eating bugs, Sussman argues. And if the ancestors of primates were
adapted to insect eating, wouldn't they have lots of insect-eating
descendants? In fact, the vast majority of living primates eat a mixed
diet including insects and plants. The few specialist insect eaters that
do exist, like the tarsier, tend to use sound rather than vision to
catch their prey.
The first flowering plants,
angiosperms, which appeared around 135 million years ago, were small
insect-pollinated shrubs and herbs. But by around 55 million years ago,
when the first true primates turn up, flowering plants had evolved into
many families of trees, and dominated the forests that covered much of
the world. In these forests, there would have been a treasure trove of
leaf buds, flowers, fruits and insects at the end of slender new
branches – a whole new feeding niche, and a powerful draw for animals
like primates, bats and birds, which evolved rapidly at this time (
see diagram).
The plants evolved nectar-rich flowers
and bigger, fleshier fruit that attracted animals like primates, and
these animals in turn pollinated their flowers, ate the fruits and
spread the seeds. The primates evolved grabbing hands and feet, and
digits with nails and sensitive pads that helped them to move around
these fine branches and manipulate the food there.
This angiosperm evolution hypothesis
not only explains why primates evolved some of their key traits, but
also the timing. "The timing is one of the best bits of supporting
evidence we have for this theory," says Magdalena Muchlinski, who
studies primate evolution at the University of Kentucky in Lexington.
Another piece of supporting evidence
comes from a 2012 study comparing the diets and ecology of hundreds of
living and extinct primates. José Gómez and Miguel Verd of the
University of Granada, Spain, found that
helping flowering plants was a recipe for success.
Fruit-eating primates that spread the seeds of the plants they fed on
were less likely to go extinct, had larger ranges and gave rise to more
new species. "It suggests that fruit eating and seed dispersal helped
fuel primate evolution and diversification," says Gómez.
All this evidence is circumstantial,
though. It doesn't prove that flowers rather than insects drove early
primate evolution. On paper, both theories have their merits.
"Cartmill's theory makes perfectly good sense," says Tab Rasmussen who
also studies primate evolution at Washington University, "but so does
Sussman's."
What was needed was hard evidence, but
there were hardly any fossils from the period in question. The primate
fossils that had been found all dated from 55 million years onwards.
These early primates looked a bit like modern-day lorises or tarsiers.
From the size of a mouse to the size of a cat, they fed on insects and
fruit. Crucially, though, they all already possessed key primate traits
such as forward-facing eyes, dextrous nailed fingers and grasping hands.
This means these key characteristics
evolved earlier, probably in the time between the dinosaurs' demise 66
million years ago and the appearance of the first true primates around
55 million years ago. Unfortunately, the fossil record from this time is
patchy, scarce and equivocal, made up largely of jaws and teeth. So
when Gingerich asked Bloch to find a skull in a block of 56-million-old
limestone, he was really hoping to find one of the "missing links" in
the primate record – a transitional fossil with a mix of primitive and
modern features. And that may be exactly what Bloch found.
Etching the rock away from fossils is a
slow process. It was several years after the discovery of the skull
before Bloch, working with Doug Boyer of Duke University in Durham,
North Carolina, found that much of the animal's skeleton was hiding
within. "It was remarkable in many ways," says Bloch, who is now a
palaeontologist at the Florida Museum of Natural History in Gainesville.
Arboreal acrobat
The fossil belonged to a species that was new to science,
Carpolestes simpsoni.
In life it was rat-sized, with a long tail. It had huge serrated
premolars, probably used to saw open fibre-rich fruits and nuts. It may
have eaten the odd insect, but its eyes were sideways facing. What it
did have, though, was grasping hands and feet, with nails on its big
toes only. With claws on its other digits,
Carpolestes would have
easily scrabbled up and down bigger branches, much like a squirrel. The
full details of the fossil were published in 2003 (
Science, vol 298, p 1564).
"It was an extraordinary specimen,"
says primatologist Mary Silcox of the University of Toronto,
Scarborough. "It was very influential in people's thinking." And the
fossil doesn't fit with Cartmill's visual predation theory as it was
originally proposed, according to which grasping hands and
forward-facing eyes should have evolved at the same time.
Instead,
Carpolestes points to a scenario
first proposed by Rasmussen back in 1990
after he spent many nights studying the woolly opossum. This arboreal
acrobat, found in the rainforests of Costa Rica, is not related to the
primates, but has evolved similar characteristics, including
forward-facing eyes and the ability to grasp. Rasmussen thinks the
marsupial evolved these traits because it behaves like the early
primates. It picks fruit on thin branches, clinging with its hands and
feet as the branches shake and pitch violently under its weight. But the
woolly opossum is an adept visual predator too, snatching moths and
other insects.
Rasmussen suggested our early primate
ancestors evolved grasping hands and feet as they climbed on slender
branches in search of fruits, flowers and insects, much as Sussman had
suggested. Later they evolved enhanced vision to catch more insects, as
Cartmill had suggested. So both ideas could be right.
Carpolestes fits nicely with
this flowers first, insects later scenario. But in 2008, theoretical
neurobiologist Mark Changizi at 2AI Labs in Boise, Idaho, threw a
spanner in the works by suggesting that the whole rationale behind the
insect predation was wrong. Animals, including predators, didn't evolve
forward-facing eyes to judge distances, he argued – it helps but there
are other ways that the brain can do this. Instead, its primary
advantage is to help animals see in environments cluttered with leaves
and branches.
Hold a finger in front of you and look
at what's behind it. With both eyes open, you can effectively see
through your finger. Close either eye, though, and part of the
background disappears. "Our eyes give us X-ray vision," says Changizi.
This "X-ray vision", however, only
works for objects narrower than the width between our eyes. So large
animals with far-apart eyes will be able to see through most branches
and leaves. If they live in a leafy environment, they will get the best
view of their world if both eyes face forwards – the increased view
ahead more than compensates for lost vision behind. Small animals like
mice don't benefit from this effect because most leaves are wider than
the distance between their eyes. They are better off with
sideways-facing eyes.
If this theory is right, Changizi
realised, the degree of overlap in the visual fields of the eyes of
animals should depend on two things: their body size (which largely
determines the distance between the eyes) and whether they live in a
leafy environment. In
a study of 319 diverse mammals,
he showed that there was a correlation between body size and overlap in
mammals living in forests, but not in uncluttered environments. So once
primates took to the trees, a relatively large ancestral primate may
have evolved forward-facing eyes to see better in forest canopies.
This fits well with the flower idea,
but would rule out the insect-hunting hypothesis. Cartmill, now at
Boston University, dismisses Changizi's challenge, pointing out that
Thomson's gazelles and cheetahs both live in grasslands, but only the
predator has eyes facing forwards. "Optic orientation in mammals doesn't
correlate with clutter," he says.
Changizi, however, says that stalking
cheetahs will be trying to see through grasses and bushes, whereas
gazelles' views will be largely uncluttered when they stand tall to
check for predators. Small predators like weasels also tend to have
sideway-facing eyes, he points out, which can be explained by his X-ray
vision hypothesis but not by the idea that stereoscopic vision is the
more important of these factors.
Put Changizi's study and the fossil of Carpolestes
together, and the flower idea looks like the clear winner. But there is
another twist to the tale. Many primatologists, including Bloch and
Sussman, think that the group Carpolestes belongs to, the Plesiadapiformes, were close cousins of the early primates and thus very similar to them.
Others think their features are so
un-primate-like that they must have been much more distant relatives "If
you put skin on them and had them run around a zoo, you wouldn't think
they look like primates," says primatologist Dan Gebo of Northern
Illinois University. If so, Carpolestes does not tell us what early primates were like after all. "The fossil is irrelevant," says Cartmill.
So who is right? The only way to
settle the issue will be to find more fossils from that vital period
that are undoubtedly those of the direct ancestors of primates. Fossil
hunters are looking, but it could take a long time and an exceptionally
patient, keen pair of eyes to spot them. "They're going to be tiny,"
says Gebo. "We're more likely to find teeth and jaws than entire
skeletons, and a jaw might only be a few millimetres long."
And, in keeping with their divided
opinions, primatologists can't decide where to look. Some, like Gebo,
favour sites in China, Europe and North America as this is where later
primates have been found. Others think the absence of early primates in
these areas suggest they originated elsewhere, so prefer to hunt in
India and Africa.
Surely, though the evidence is out
there, if we can only find it. Perhaps even as you read this, the rock
containing that vital missing link is being painstakingly etched away by
another graduate student. In the meantime, the next time you look at a
flower, remember that blossoms may have made us what we are today.
This article appeared in print under the headline "Flower child"
Helen Pilcher is a freelance writer based in the UK. Follow her on @HelenPilcher1
