Not since an asteroid smacked into Earth to end the reign of the dinosaurs 65 million years ago has the evolutionary future of life on the planet been rewritten as extensively or as suddenly as it is being rewritten today.
By the middle of this century, many biologists estimate, human activities from urban sprawl and deforestation to overfishing will have erased up to 30 percent of the species inhabiting the planet.
Millions of those plants, animals, insects, and microbes never will have mushroomed under a field researcher's microscope. Their potential value for the ecological role they play or the useful chemicals they produce and place in history will go unassessed.
This prospect is adding urgency to a new effort to build a comprehensive "family tree" of life on Earth from bacteria to whales and all their fossilized ancestors. In the process, researchers hope to pull together an Internet-based catalog of Earth's existing species.
An exhaustive "tree" establishing the evolutionary relationships of organisms, and a catalog placing them in their ecological niches, are expected to become powerful tools for guiding conservation efforts, discovering new chemical compounds for human use, and even investigating suspected acts of bioterrorism.
Researchers add that the tree of life will yield new insights into evolution itself, answering such questions as how often photosynthesis emerged, and whether life evolved only once. And, they say, the prospect of being able to address these questions marks a new chapter in the life sciences.
"We are at a historic turning point in biology," says Harvard University entomologist Edward Wilson.
He explains that biologists until now have been reductionists, probing organisms down to the level of individual genes. This has culminated in various projects to map and sequence the genomes of organisms ranging from bacteria to humans.
"Now biology is entering its synthesis phase," he continues. "We're trying to put it all back together. This puts the emphasis on complexity, on self-assembly, on interdisciplinary work, and on the lateral spread of studies of an organism" among scientists worldwide.
This interdisciplinary approach is one of the distinguishing features of the new tree-of-life effort, says Quentin Wheeler, director of the environmental biology division at the National Science Foundation in Arlington, Va.
"Until now, this sort of work has been done piecemeal, with individual scientists slogging on their own," says Dr. Wheeler.
The NSF is spending up to $10 million this year and is seeking $12 million for next year to begin the process of mapping "significant branches of the tree," he says. The agency's tree-of-life initiative, which could expand into an international effort following meetings with biologists in Europe later this year, could take up to 15 years to complete.
Although humans and their ancestors have long been classifying organisms out of eat-or-be-eaten necessity, scientists moved into the field in a major way following Darwin's publication of "On the Origin of the Species" in 1859.
Yet only in the 1960s, when scientists standardized their approach to organizing organisms, did tree-building take off, according to Joel Cracraft, an ornithologist at the American Museum of Natural History.
Today, "out of the 1.7 million species we know of, up to 70,000 have been looked at," Dr. Cracraft says. "Eighty percent of what we know of the history of life, we've learned in the last decade."
And plenty of work remains to be done. Biologists estimate that from 10 million to 15 million additional species await discovery.
"There are huge areas on the tree we know nothing about," laments Terry Yates, a University of New Mexico biologist who is widely acknowledged as a prime mover behind the current tree-of-life effort.
Some of those areas lurk in museum drawers.
Oliver Zompro, a graduate student at the Max Planck Institute for Limnology in Plön, Germany, was studying a sticklike insect preserved in a collection in London. He established that the specimen, which came from Africa, represented a new insect order that dates back at least 45 million years. He reported his findings in April in the journal Science.
Two trends have converged to open the possibility of building a comprehensive tree of life, researchers say.
One is the emergence of the affordable computing horsepower needed to compare and find patterns in large amounts of data about, for instance, habitats, fossil information, and physical characteristics.
The other is the growth of molecular tools to help establish relationships among organisms. These would have eluded scientists focusing primarily on what creatures, plants, or microbes look like, or the features they share.
Prominent among these is biologists' ability to sequence entire genomes of organisms. Researchers have learned to use sequences of DNA, which carry the genetic blueprint, and RNA, the molecular contractors that take DNA's information and begin the building process, to help establish evolutionary relationships between and among organisms. For example, such techniques have helped establish that as a group, fungi are more closely related to animals than to plants.
The use of molecular techniques to help establish lineages can be a matter of life and death, scientists say.
During a recent conference on "Assembling the Tree of Life" at the American Museum of Natural History, one biologist noted that the efficacy of antivenin for bites from Australian brown snakes depends on how closely related the antivenin's source is to the species that did the biting. The various species of brown snake are hard to distinguish at a glance, he said.
Researchers are quick to note some of the social and economic benefits they hope to derive from building a comprehensive tree of life.
"Nature has provided us with billions of years of free R&D," Dr. Yates says. The tree would allow scientists "to make an intelligent search for very specific products," from new pharmaceuticals to "natural" bug repellents.
But over the long term, a tree's greatest value may lie in its potential for guiding conservation strategies as the international community strives to preserve the planet's biodiversity in the face of what some researchers term the planet's sixth extinction event.
Unchecked, human activities are conducting an enormous, uncontrolled evolutionary experiment, say biologists Andrew Knoll of Harvard University and Norman Myers of Oxford University in England.
Humans "are 'deciding' on evolution's future in virtually a scientific vacuum deciding all too unwittingly, but effectively and increasingly," the two scientists have written.
The fossil record suggests that it takes roughly 5 million years for earth's biosphere to recover from a major extinction event, they say. While no one can forecast what may happen to a given species at the end of that period, the two add that biologists can provide a sense for how reduced diversity could affect the broader course of evolution.
For example, they suggest that large mammals, which have relatively slow reproductive cycles, could give way to species that adapt more quickly to changes. This could lead to an increase in the number of ecosystems dominated by organisms that today we would consider pests and weeds. In addition, diversity could be expected to increase among species that thrive in human-affected ecosystems.
In total, while evolution certainly would continue, it could yield a more homogenized set of ecosystems with much less diversity overall than exists today.
Conservation efforts that once focused on high-profile species have largely shifted toward projects that preserve ecosystems inhabited by endangered species.
Yet, researchers say, conservationists may need to take yet another conceptual leap by designing projects in ways that preserve "the evolutionary processes at risk."
Drs. Myers and Knoll posit that it is more important to maintain the "potential for diversity generation" and the "functional groups that increase the potential for recovery" than individual species.
"What do we save that preadapts the world to rediversify?" asks the NSF's Wheeler.
He suggests that the importance of preserving old-growth forests in northern California may rest at least as much on preserving his "favorite poster child" a group of primitive wingless insects known as bristletails as it does on preserving the spotted owl.
One species of bristletail in particular was known only from fossils and was thought to have been extinct for millions of years. Then in the 1960s, Wheeler says, live specimens of these insects were found in the old-growth forests of northern California's coast.
New molecular analyses suggest that the "fossil" species is the sister to the line that begat all 800,000 winged-insect species known today. The creature may represent an evolutionary branching point that could in the future repeat its diversification.