Diving into the History of Life

Something small, round, and faintly iridescent caught my eye while walking along the edge of a sandy beach on Florida's Gulf coast. I lifted it to within range of the hand lens that I often sling on a cord around my neck-a habit acquired years ago during a botany class. "Never leave home without one!" the professor said. "Most of life on earth is microscopic. Think small!" Sage advice, I soon learned. Humans, like elephants, horses, whales, and trees, are giants on a planet largely populated with creatures more diminutive than houseflies.

Aiming the lens, I magnified the tiny sphere and saw within a miniature version of the dinnerplate-sized mahogany-colored horseshoe crabs that had crowded the shoreline a few weeks earlier. This one was translucent and seemed extremely fragile, but the tiny egg resting on my fingertip contained a highly durable, tried-and-true 400-million-year legacy. Horseshoe crabs preceded the era of dinosaurs by more than 100 million years and persisted through changes that devastated much of the rest of life on earth. Four species remain, each bearing the weight of a distinguished ancestry, including clues to the nature of many long-extinct relatives. They represent but one of numerous categories of life that occur only in the sea.

Diving into the sea (something I do often, given the faintest of excuses) is like diving into the history of life on earth. Nearly all of the major divisions of plants and animals have at least some representatives there, from minute microorganisms, to sponges and jellies, worms, bryozoans, starfish, molluscs, and plants that have no counterpart in any terrestrial place. Even the marshy sand beneath my feet, awash with salt water, provided home for thousands of small creatures representing at least twenty phyla of animals and several divisions of plants. In a handful of such sand, the two aspects of life that I find most amazing can be found: the enormous diversity of life coupled with underlying similarities. Each and every sand-dweller is unique-just as each moss or fern frond and every fish, cat, dog, horse, and human is a one-of-a-kind individual; yet, the basic chemistry and fundamental genetic makeup of the strands of blue-green algae laced among the grains, the sand fleas, the polychaete worms, the nematodes-and my hand-are much the same.

Using scuba, I have explored the upper edge of the sea, revelling in encounters with fellow vertebrates-fish, birds, mammals-that have mastered the sea with much greater finesse than my species. And, using technology devised by clever human minds, I have dived deeper than any bird or whale can go, into the great mid-water space that embraces most of the planet. Each time is spiced with excitement that comes with exploring the unknown. Ninety-seven percent of earth's water is ocean, and more than 97% of that vast, three-dimensional living space remains unexplored. Think of it! The sea shapes the character of earth, governs climate and weather, regulate temperature, and comprises much of the biosphere-yet it remains largely unknown. It is not, however, untouched. Everywhere, the changes brought about by humankind are evident. I have counted beer cans in 4000 meters depth, dodged abandoned fishing nets strung with dead and dying creatures, and wondered at the enormous magnitude of fish, crabs, shrimp, squid, and other wildlife removed from the sea as well as the enormous magnitude of toxic materials added during my lifetime. The consequences of such actions do not bode well for the future of life in the sea-or on land.

With great care, I returned the young crab to the sand, mindful that the success or failure of its kind depends largely on the actions of mine. But I believe the greatest threat by far to their future-and ours-is ignorance. With knowing comes caring, and with caring, the hope that we'll discover how to strike a balance with the natural systems that sustain us and thus achieve an enduring place for humankind (and horseshoe crabs) on a planet that got along without us for billions of years. Surely, it could do so again.