Recently I’ve been trying to imagine a world without leaves. It’s not easy to do at this time of year, when the trees around my house turn my windows into green walls. But a paper published on-line today at the Proceedings of the National Academy of Science inspires some effort. A team of English scientists offer a look back at Earth some 400 million years ago, at a time before leaves had evolved. Plants had been growing on dry land for at least 75 million years, but they were little more than mosses and liverworts growing on damp ground, along with some primitive vascular plants with stems a few inches high. True leaves–flat blades of tissue that acted like natural solar panels–were pretty much nowhere to be found.

It’s strange enough to picture this boggy, bare-stemmed world. But it’s stranger still to consider that plants at the time already had the genetic potential to grow leaves. Some species of green algae–the organisms from which plants evolved–were growing half-inch leaf-like sheets 450 million years ago. Tiny bud-like leaves have been found on 400 million year old plant fossils. Despite having the cellular equipment necessary to grow leaves, plants did not produce full-sized leaves in great numbers until about 350 million years ago. When they finally did become leafy, the first trees emerged and gave rise to the earliest forests. Leaves have dominated the planet ever since. They capture enough carbon dioxide to make millions of tons of biomass every year, and as roots suck up water, trillions of gallons evaporate through them.

Why did leaves take 50 million years to live up to their genetic potential? Apparently they had to wait.

Plants, the researchers point out, take in carbon dioxide through elaborate channels on their surface called stomata. Living plants can adjust the number of stomata that grow on their leaves. If you raise them in a greenhouse flooded with carbon dioxide, they will develop significantly fewer stomata. That’s because the plants can gather the same amount of carbon dioxide they need to grow while allowing less water to evaporate out of their stomata.

Geological evidence shows that 400 million years ago, the atmosphere was loaded with carbon dioxide–about ten times the level before humans began to drive it up in the 1800s. (It was 280 parts per million in the early 1800s, 370 ppm today, and is predicted to rise to 450 to 600 parts per million by 2050. In the early Devonian Period, it was around 3000 ppm.) Consistent with living plants, the fossil plants from the early Devonian had very few stomata on their leafless stems.

Why didn’t these early plants grow lots of leaves with few stomata? If they did, they could have grown faster and taller, and ultimately produced more offspring. But the scientists point out that a big leaf sitting in the sun risks overheating. The only things that can cool a leaf down are–once again–stomata. As water evaporates out of these channels, it cools the leaf, just as sweat cools our own skin. Unable to sweat, early Devonian leaves would have been a burden to plants, not a boon.

About 380 million years ago, however, carbon dioxide levels began to drop. Over the next 40 million years they crashed 90%, almost down to today’s levels. The decline in carbon dioxide brought with it a drop in temperature: the planet cooled enough to allow glaciers to emerge at the poles. In the paper published today, the scientists describe what happened to plants during that time. Two different groups of plants–ferns and seed plants–began to sprout leaves. As years passed, the leaves became longer and wider. And at the same time, the leaves became increasinly packed with stomata. From 380 to 340 million years ago they became eight times denser. It seems that the drop in carbon dioxide and temperature turned leaves from burden to boon, and the world turned green.

It’s possible that plants themselves may have ultimately been responsible for the emergence of leaves. Before leaves evolved, roots appeared on plants. Unlike moss and liverworts, which can only soak up the water on the ground, plants with roots can seek out water, along with other nutrients. Their probing eroded rocks and built up soil. The fresh rock that the plants exposed each year could react with carbon dioxide dissolved in rainwater. Some of this carbon was carried down rivers to the ocean floor and could no longer rise back up into the atmosphere. In other words, roots pulled carbon dioxide out of the atmosphere and made it possible for leaves to evolve. The evolution of leaves in turn led to the rise of big trees, which could trap even more carbon, cooling the climate even more. Clearly, we are not the first organisms to tinker with the planet’s thermostat.

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