Life in the balance: our failure to recognize our connection with the global ecosystem lies behind the biodiversity crisis facing our planet today

From ostriches to sparrows, birds find their closest relatives among the extinct sauropod dinosaurs, which included the likes of Tyrannosaurus rex and the huge brontosaurians. Most birds, like bats, fly; lacking teeth, birds pierce, bite, and crush their various foodstuffs with their specialized, horny beaks. Their feathers, like mammalian hair, were derived from primordial reptilian scales, perhaps first as a means of regulating body temperature and later as indispensable adjuncts for powered flight.

Mammals

Insectivores--moles, shrews, hedgehogs, and the like--are insect (and earthworm) eaters. Bats (Order Chiroptera) presumably derived from early insectivores and belong to one of the most diverse groups of mammals, one with more than 986 known species.

Rodents, with some 1,814 known species, are distinguished by ever-growing incisors, allowing them to gnaw away at their vegetable foodstuffs. They live in an amazing array of environments, from the Arctic tundra (lemmings) to the depths of the tropical rainforest (South America's capybara--the world's largest rodent.)

Humans

We, species Homo sapiens, are vertebrate animals--although of rather a peculiar sort. Unlike all other organisms, most humans no longer live within the confines of local ecosystems. Through our self-awareness, our cognition, and our capacity to use language, we have collectively devised an elaborate behavioral system handed down through the generations by learning rather through genes. This is our unique adaptation, through which we perform the usual gamut of animal functions.

We are members of the Order Primates, allied through a long evolutionary sequence with the great apes, Old and New World monkeys, and the "lower primates,' including lemurs and lorises. We are among the more primitive of mammalian groups, stretching all the way back across the Mesozoic-Cenozoic boundary to the Cretaceous, when true mammals first began their evolutionary radiation.

Viruses

What of viruses? Aren't they alive as well? In a word, no. Viruses have RNA but are utterly unable to reproduce on their own. Although much smaller and simpler than bacteria (suggesting that they are an even more primitive form of life), viruses are parasites, utterly dependent on the DNA of living creatures, whose cells they invade, take control of, and use to replicate themselves. Thus, viruses--whatever they are--must have made their appearance after bacteria and eukaryotes.

Fantastic as it may seem, I think there is much to the suggestion (from microbiologist Lynn Margulis) that viruses are escaped snippets of the genetic code of other organisms. Viruses do not form a coherent natural group. Some--such as those that cause polio, AIDS, or flu--may be more closely related to us than they are to, say, the tobacco mosaic virus. Important as they nonetheless are to the well-being (or lack thereof) of the world's true organisms, viruses stand to the side in the spectrum of life.

COPYRIGHT 1998 Natural History Magazine, Inc.
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