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FALLING levels of atmospheric carbon dioxide gave primitive land plants the cue to spread their leaves and soak up the sun.

In the early Devonian period, 400 million years ago, leaves were tiny stick-like projections called microphylls, and were far less efficient at photosynthesis than the broad, green solar panels sported by many modern plants. Those ancient plants had the genes and physiological machinery to produce big leaves, but they didn鈥檛 do so for at least 20 million years.

David Beerling and Colin Osborne at the University of Sheffield, UK, and William Chaloner at the Royal Holloway, University of London wondered whether high levels of CO₂ at the time were what held back leaf development, and devised a computer model to simulate leaf behaviour.

In the early Devonian, the level of atmospheric CO₂ was 10 times today鈥檚, and plants required far fewer pores, or stomata, to take in the gas. But there was a downside to having so few stomata, their model showed. Stomata also act like sweat glands, cooling the leaf by releasing water. If the leaves had grown much larger, they would have soaked up more of the sunlight, but would not have had enough pores to prevent overheating. 鈥淚t really jumped out at us how easy it is to cook a leaf,鈥� Osborne says.

The drop in CO₂ levels 370 million years ago might have forced plants to increase the density of stomata, which would in turn have allowed them to grow large leaves while keeping cool.

To test the idea, the researchers looked at the fossil record and measured the leaf area of 300 plant species, as well as the number of stomata on a few rare specimens with intact outer cuticles. Sure enough, they found that the rapidly dropping global CO₂ levels were matched by an increase in pore density, and increasing leaf area (Proceedings of the National Academy of Sciences, vol 101, p 10360).

It is the first convincing evidence that explains the time lag in leaf evolution, says Paul Kenrick, a palaeobotanist at the Natural History Museum in London.