Animals that give seeds a helping hand

SIR DAVID ATTENBOROUGH: 47 million years ago plants had begun to develop large and fleshy fruits. It was one of the ways in which they had adapted to the new, dense forest environments.

Many early plants relied on the wind to distribute their seeds. But in the forest, there's little or no wind, so they had a problem. They solved it by recruiting the help of birds, and they did that by wrapping their seeds in an edible, sweet flesh—fruit.

Birds carried the seeds in their stomachs, and eventually deposited them elsewhere in the forest. The tree-living primates were ideally placed to exploit this cozy arrangement. But to take full advantage, they needed to improve their vision.

During the age of the dinosaurs, when the mammals were largely nocturnal, they had developed better night vision but sacrificed the feature not needed in the dark: the ability to see colour. Today, most mammals still see the world largely in black and white. But the reptiles, and their cousins, the birds, retained excellent colour vision. And the fruit-bearing plants had evolved a signalling arrangement to match.

There's no point in having your seeds distributed before they're fully formed. So the plants evolved a colour coding system to show when that was. To spot a flash of red colour in amongst the green foliage is easy for a bird or a reptile. But for a mammal, with their night-time vision, red and green are indistinguishable. Then, remarkably, some of the primates managed a feat no other mammal has achieved—they put evolution into reverse, and reacquired colour vision.

The common ancestor of this monkey, and of me, lived up in the trees in the daylight. And they quickly evolved the ability to see colour, and therefore to know which was ripe and which was unripe fruit, and so take advantage of the system that had already been worked out between the birds and the plants. Let's just see what she thinks about that. Which of those do you like? There's it!