The Emerald Planet

From the hottest deserts to the coldest Arctic tundras, plants are everywhere. Their diversity is staggering – just compare the smallest dandelion to the tallest fir – but so, too, are their similarities. You probably know that nearly all plants convert carbon dioxide and sunlight into energy through a process called photosynthesis. And there’s one key evolutionary development that makes photosynthesis far more efficient: leaves. 

The key message here is: Plants developed leaves to cope with a decrease in carbon dioxide, thereby kickstarting the evolution of animals and insects.

The first plants were completely leafless – and for the first 40 million years of their history, they did just fine. For decades, this remained a mystery to scientists; even in evolutionary terms, this is a very long time. But the latest research reveals a missing piece of the puzzle: the levels of CO2 in the atmosphere. 

Plants absorb CO2 through tiny pores called stomata, located on the surface of their leaves. The number of these stomata varies with CO2 levels. When there’s plenty of it, plants don’t need quite so many pores. And when CO2 becomes more scarce, the number of stomata increases. 

So plants are constantly adjusting to the changing environment. Older leaves can even communicate with younger ones, telling them how many stomata to make. And understanding stomata can explain how leaves evolved. 

Approximately 375 million years ago, CO2 levels in the atmosphere plummeted. This meant that plants needed more stomata in their leaves to capture the same amount of CO2. As a result, their leaves got bigger. Plants adjusted to the new environment and became abundant. That, in turn, paved the way for an evolutionary boom of animal and insect species.

But what brought about this dip in CO2 levels to begin with? Well, it’s plausible that plants themselves were responsible. Plants can disrupt what’s called the long-term carbon cycle – a process that manages the exchange of CO2 between rocks, oceans, and the atmosphere. 

Fungi and the roots of plants actively removed CO2 from the atmosphere. As carbon dioxide levels continued to drop, plants needed ever bigger leaves. This set off a feedback loop, which spurred on the triumphant march of leafy plants across the planet.