How flower power paved the way for our evolution - New Scientist

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.

So Sussman came up with another idea. Inspired by his studies of modern Madagascan lemurs that regularly tap nectar-rich flowers for food, Sussman and palaeobotanist Peter Raven proposed that primates evolved in tandem with flowering plants.

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