How Did Life Invade the Land?

A compelling subplot in the long tale of life's history is the story of life's invasion of land after more than 3 billion years of a strictly watery existence. In moving to solid ground, organisms had many obstacles to overcome. Life in the sea provides buoyant support, but on land, an organism must bear its weight against the crushing force of gravity. The sea provides ready access to life sustaining water, but a terrestrial organism must find adequate water, sea-dwelling plants and animals can reproduce by means of mobile sperm and/or eggs that swim to each other through the water, but the gametes of land-dwellers must be protected from frying out.

Despite the obstacles to life on land, the vast empty spaces of the Paleozoic landmass represented a tremendous evolutionary opportunity. The potential rewards of terrestrial life were especially great for plants. Water strongly absorbs light, so even in the clearest water; photosynthesis is limited to the upper few hundred meters of depth- and usually much less. Out of the water the dazzling brightness of the sun permits rapid photosynthesis. Furthermore, terrestrial soils are rich storehouses of nutrients, whereas seawater tends to be low in certain nutrients, particularly nitrogen and phosphorus. Finally, the Paleozoic sea swarmed with plant-eating animals, but the land was devoid of animal life. The planets that first colonized the land would have had ample sunlight untouched nutrient sources, and no predators.

In moist soils at the water's edge, a few small green algae began to grow, taking advantage of the sunlight and nutrients. The did not have large bodies to support against the force of gravity, and living in the thin layer of water on the soil, they could easily obtain water. About 400 million years ago, some of the algae gave rise to the first multicellular land plants. Initially simple, low-growing forms, land plants rapidly developed solutions to two of the main difficulties if plant life on land: obtaining and conserving water and staying upright despite gravity and winds. Waterproof coatings on the above ground parts reduced water loss by evaporation, and rootlike structure delved into the soil, mining water and minerals. Specialized cells formed tubes called vascular tissues to conduct water from roots to leaves. Extra-thick walls surrounding certain cells enabled stems to stand erect.

Question for you: why are today's ferns and club mosses so small in comparison to their giant ancestors? Science is fun.