Leaves naturally turn yellow in the fall, when green chlorophyll fades to reveal the golden pigments already in them.

But it takes energy to turn leaves red, and not all trees can do it. Why?

Scientists have three hypotheses:

The first is sunblock. Red pigments protect the leaves better than yellow, allowing them to stay on branches longer, so that trees can harvest more nutrients before they fall.

The second has to do with insects. In autumn, aphids feed on tree leaves.

Researchers found that aphids are five times more attracted to yellow than red, so the red pigments may keep aphids away.

The third is related—but geologic. More than a hundred tree species produce red leaves in North America and East Asia, but only a few do in Europe.

During the last glacial advance, as ice descended southward, trees and animals had to migrate ahead to survive.

In America and Asia, with no east–west mountain ranges to block them, trees migrated easily—but so did their insect pests, encouraging the trees to produce red leaves.

In Europe, east–west mountain ranges blocked tree and insect migration, causing both to go extinct locally. Without those pests, most trees that repopulated those regions didn’t develop red pigments.

The real reason may be a combination, or something else entirely—a reminder that science is a process, not an outcome, and there is much left to discover.