Dinosaurs must employ a wonderful public relations firm. Although they vanished from the Earth 65 million years ago, they continue to fascinate every generation of children and a surprising number of adults. Dinosaur toys are perennial favorites. "Jurassic Park" and "The Lost World" were smash hits at the box office.

Even in manner of their departure, the dinosaurs have been the source of excitement and much intellectual argument. For many years the prevailing theory argued that a change of climate was the reason for their extinction. That changed in 1978, when Luis and Walter Alvarez, following a suggestion of a colleague, Chris McKee, proposed that the dinosaurs had died quickly rather than slowly when an asteroid about ten kilometers across hit the Earth. The energy released by such an impact would be like a hundred million hydrogen bombs, exploded simultaneously. The effect would be the disintegration of the asteroid, which, along with many cubic kilometers of the Earth's crust, would have been thrown high into the atmosphere. That dust would have stayed aloft for six months or more, halting photosynthesis in plants, preventing plant growth, and thereby starving to death all the larger animals including the dinosaurs, the flying reptiles, and the largest marine reptiles.

The Alvarez proposal seems increasingly plausible as an explanation. However, it leaves unmentioned an important fact: the vanishing of the dinosaurs, in what is known as the K/T or Cretaceous/Tertiary extinction, may be the best-known species loss in Earth's history, but it was by no means the only or the largest one. (K/T, rather than the more logical C/T, because geologists refer to geological strata using a single letter, and C was already taken for the Carboniferous Period, which ended about 140 million years before the Cretaceous Period began.)

It is not possible to map the earliest species extinctions. The Earth is about 4.6 billion years old, and life has been present for a surprisingly big piece of that time, maybe as much as 3.8 billion years. However, the earliest life was single-celled and soft-bodied, and left little evidence of either its presence or its absence. The first signs of a major loss of species, or "mass extinction," come from about 570 million years ago, at a time known as the Precambrian-Paleozoic boundary. This was followed by the "Cambrian explosion," in which a vast new variety of plants and animals appeared on the scene. These strange and fascinating creatures, most of them long gone, are described in detail in Stephen Jay Gould's book, "Wonderful Life."

Another mass extinction occurred later in the Paleozoic era, about 380 million years ago. At this time in the Late Devonian, on the boundary between the Frasnian and Famennian Ages, coral reefs and their numerous associated marine species took a particularly hard beating.

However, neither those two Paleozoic extinctions, nor the death of the dinosaurs, forms the most catastrophic event in the fossil record. That distinction belongs to what is known as the Permian-Triassic (or sometimes the Paleozoic-Mesozoic) extinction, which took place about 225 million years ago. In that global crisis, an estimated 95 percent of all living species died out. The Earth had to be repopulated by the one in twenty that remained, which fortunately included our own ancestors.

Although these have been the most disastrous periods for life, there is plenty of evidence for other and smaller extinctions. It is natural to ask if all these events are related. Was the asteroid or comet that struck Earth 65 million years ago, near Mexico's Yucatan Peninsula, an isolated incident, or did it share a common cause with the other major extinctions?

An analysis of the times when mass extinctions occurred suggests that one comes along about every 26 million years. The statistical strength of this result is not good, but it is natural to ask if we can think of any physical event that might have such a period. One proposal is for a supposed "dark companion" to the Sun. This object, named "Nemesis" by the theory's originator, Rich Muller, is gravitationally bound to the solar system but in a very large orbit. Most of the time Nemesis is more than a light-year away from the Sun and has no effects on us. However, it returns to our vicinity every 26 million years, where it disturbs the vast reservoir of comets in a region known as the Oort Cloud. Some of these fall in closer to the Sun, to become the species-extinguishing specimens that hit the Earth.

The whole idea of Nemesis is highly controversial, and I have to say that I find the arguments for it unpersuasive. However, we do not need to invoke Nemesis to discover a great extinction of species in our own future. In fact, we do not need to look to the future at all. We merely need to look around us, at what is happening on the Earth today.

An estimated million species a year are vanishing from the planet. To put that in perspective, a million species a year means two species are disappearing every minute. In the time that you have taken to read this column, another handful have vanished. The agent for their demise is not some cosmic impact. It is our own energetic activity, as we drain swamps, dam rivers, clear rain forests, build pipelines, irrigate deserts, and fill the rivers and seas with our waste products.

Most of the species that today vanish forever do so before we have even identified and named them. They are obscure and often tiny fungi, beetles, worms, bacteria, algae, spiders, crustaceans, mollusks, copepods, and coral polyps. Their importance to Earth's biosphere as a whole is quite unknown.

We understand neither what they do, nor what will happen if they vanish. We have, however, discovered in recent years that Earth's life forms constitute one gigantic, interdependent, interlocking entity. It is impossible to remove one species or family without a domino effect spreading through the whole, probably annihilating other species on the way. We have mathematical models to study the propagation of changes in plant and animal populations, but they are nowhere near big enough or sophisticated enough to handle a case where a thousand species, still less a million, vanish all at once.

Which forms will be alive, which will be dead, a few centuries from now? We simply do not know. What if one of the dominoes destined to fall during the current mass extinction turns out to be Homo sapiens?

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