How and why did leaves evolve? F. A. Jones 03.23.01 Most plants that you see today have leaves, that much seems simple. However, the fossil record shows that the abundant green foliage now common on trees, shrubs, and herbs was not present when plants began to appear on land 400 million years ago. In fact, for the first 40 million years after plants first appeared on land, they didn’t have any leaves at all. Most plants at this time resembled leafless stems. Why, then, did leaves suddenly appear in the fossil record 360 million years ago?    Researchers have suggested that leaves could only have evolved after a steep drop in carbon dioxide concentrations in the atmosphere. Leaves can soak up carbon dioxide through pores called stomata. Carbon dioxide is a necessary ingrediant in the process of photosynthesis to make sugars. When plants first invaded land, carbon dioxide was abundant in the atmosphere and plants had few stomata. Any leafy structures that evolved at this time would have suffered from overheating, as the leaves had no active way of cooling off. However, as carbon dioxide levels dropped, plants required more stomata to get more carbon dioxide. Greater numbers of stomata also meant that plants would lose more water through these pores, thereby cooling the leaf. Therefore, scientists have suggested that the evolution of leaves was made possible through a drop in carbon dioxide and an increase in the number of stomata in leaves. These stomata would have allowed larger leaf areas to evolve, thereby increasing the efficiency of photosynthesis. H. Fountain "How Plants Got Leaves" New York Times, March 20, 2001.  ©2001 The Aurora Collection, Inc. All Rights Reserved. Site Development by: Interactive Multimedia. Inc.