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Why are there no lobsters on lands or bats at sea? The answer appears to be a biological version of beginner's luck
Natural History, Feb, 2002 by Geerat J. Vermeij
I can't help noticing them. With every sweaty step I take along a trail in the magnificent rainforest on Panama's Barro Colorado Island, insects of every description make their presence known. The piercing shrieks of cicadas high overhead drown out the territorial calls of birds. To one side of the trail, a pair of orchid bees, wings vibrating in a low, almost threatening pulsating buzz, engage in an elaborate aerial mating dance. The leaves of the understory shrubs, lianas, and sapling trees bear the unmistakable signs of damage by hungry insects. Some are mere skeletons of veins, all the intervening leaf tissue having been nibbled away by caterpillars. Ants burdened with loads of leaf fragments march toward their underground fungal gardens. And the insects are hardly alone. Lizards scurry in the leaf litter at my feet, monkeys feed noisily in the branches above nay head, a vine snake makes its sinuous way toward an unsuspecting small bird. Like the insects and plants, all these creatures are occupied with the business of extracting a living from the land.
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Just a few miles away, on the warm rocky shores and mudflats of the Bay of Panama, life-forms are markedly different. There--where the sea has as much influence on living organisms as terra firma does--barnacles and bivalves and bryozoans filter food particles out of the water, snails and sea urchins and octopuses and crabs consume everything from decaying matter to other animals, and tiny corals and seaweeds photosynthesize on tide pool rocks.
Offshore, of course, the contrast with life on land is even greater. All photosynthesis in the sea is carried out by microscopic plants that drift near the surface and find their way up the food chain not only to such typically marine animals as sharks and squid but ultimately also to air-breathing whales and albatrosses.
That the cast of characters living on land is so different from the one inhabiting the sea is such a familiar observation that we rarely stop to ask why this is so or how the differences are maintained. Yet such everyday questions provide some of the most interesting puzzles to the naturalist, and the answers can yield deep insight into how nature works.
A little history puts the problem in perspective. All the available evidence--derived from studying the evolutionary relationships of all kinds of extinct and living organisms--indicates that plants and animals originated in the sea. Multicellular marine organisms may have existed as early as 1.7 billion years ago, and plants identifiable as red algae were certainly growing 1.2 billion years ago. When marine animals appeared remains a matter of controversy, but an estimate of 700-800 million years ago seems reasonable. By Middle Ordovician times, about 460 million years ago, fungi (the oldest recognized multicellular terrestrial organisms) had colonized the land. During the next 140 million years, other types of organisms invaded terra firma and gave rise to vascular plants, millipedes, mites, insects, spiders, scorpions, land snails, and early amphibians and reptiles.
A look at the distribution of the major groups of plants and animals alive today is instructive. (By "group," I mean a clade, an evolutionary branch consisting of an ancestor and all its descendants. A major group is a large and typically old clade, such as insects or vertebrates.) Most of these groups live either in the sea or on dry land, but not in both. A small minority occupy freshwater habitats. Very few groups have made the transition from water to land or from land back to water, and groups that have moved between marine and terrestrial habitats are especially rare. This may seem surprising, since land and sea are both so full of life, but the facts bear out the statement. Mollusks, for example, have occupied the sea for more than 500 million years--seemingly ample time to make the move--but only nine groups of snails, and no clams or cephalopods (the group containing squid and octopuses), have ventured onto dry land. With an equally long history in the sea, arthropods (a huge group of invertebrates including insects, crustaceans, and spiders) have made the transition to land at most nine times. Green plants may have done it only once. Lobsters, sea stars, corals, bryozoans, brown algae, and a host of other marine groups never made it.
Once on shore, of course, some colonizers were spectacularly successful. All the plants in your neighborhood, for example--from the loftiest tree to little spring pansies--may have evolved from a single ancestor. Groups of animals and plants that made it to land prior to about 300 million years ago were especially prolific in their new surroundings. There are now hundreds of thousands of species of insects and green plants, and tens of thousands of mites, spiders, and pulmonate land snails (those endowed with lungs). Later arrivals, such as sowbugs, crabs, hermit crabs, and some other land snail groups, have remained minor elements of the terrestrial fauna.
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