Fossils Prove Life Came About Faster
By MALCOLM W. BROWNE
Published: April 30, 1993
Fossil microorganisms discovered in Australia by an American scientist prove that life was already thriving and diversified 3.485 billion years ago, leaving a much narrower window than previously thought for life to develop.
The microorganisms, representing 11 distinct types, are more than 1.3 billion years older than any comparable fossil group ever found. This dramatically curtails the time available for life to have evolved naturally on earth, and could focus more attention on the disputed hypothesis that life originated elsewhere in the universe and somehow reached earth from afar.
At the least, the new findings are likely to prompt a re-examination of rates at which species are presumed to have evolved on the early earth.
In a report to be published today in the journal Science, Dr. J. William Schopf of the University of California at Los Angeles describes 11 distinct types of microorganisms found in an unusual rock formation in northwestern Australia. The microscopic fossils were embedded in tiny mineral grains, which were encased in another type of rock that scientists were able determine formed 3.485 billion years ago. Single-Celled Organisms
The grains in which the fossils were found are almost certainly older than the rock encasing them, but Dr. Schopf has been unable to determine just how old that might be.
All the Australian fossils are of single-celled organisms in the form of microscopic filaments less than a hundredth of an inch long. Their worm-like shapes, Dr. Schopf said, resemble those of certain modern bacteria and cyanobacteria -- the most primitive organisms capable of photosynthesis.
In photosynthesis, the process that fuels most plants, the energy of sunlight is used in synthesizing carbohydrates and other complex biological building materials from the carbon dioxide in the planet's air and water.
These pioneers of life on earth may have founded the lineage that many eons later led to both plants and animals, including humans. Because of their similarity to modern cyanobacteria, Dr. Schopf believes, the organisms may already have developed photosynthesis as a source of fuel -- a surprising achievement for organisms that had so little time to evolve.
Estimates of the age of the earth vary, but many scientists believe the planet solidified in its present form about 4.6 billion years ago.
Scientists know that the moon, which formed at the same time, was peppered by devastating meteor impacts during the infancy of the solar system, and the earth must have been subjected to the same bombardment. Even if life began during this early period, it would have been quickly extinguished by the fiery impact of meteors.
Many geologists believe that life could not have achieved a permanent toehold on the earth until the meteor bombardment subsided about 3.9 billion years ago. Hitherto, the dating of the earliest known life form allowed theorists more than one and a half billion years in which to account for the presumable development of living cells from the soup of chemicals covering the primordial earth. The new finding shrinks that window to a mere 500-million-year time span.
"It may seem astonishing that life could have reached such a degree of complexity and diversity so long ago," Dr. Schopf said in an interview, "but I don't think biologists will be surprised for long."
Most scientists believe that terrestrial life originated on the earth itself, arising from a primordial soup of amino acids and other carbon-based chemicals by some yet-to-be-explained mechanism. But a few scientists have suggested that life may have drifted to the earth in the form of spores or single-celled organisms from some other place in the galaxy or universe.
If life began elsewhere, it would not have required a long period to evolve microorganisms like those found in Australian rocks, and fairly advanced organisms might have appeared soon after the earth cooled down.
Among those who have proposed the "panspermia" theory, that life arrived on earth in the form of interstellar microorganisms, are Sir Fred Hoyle, a British theorist, Dr. Francis Crick of the Salk Institute in San Diego, , co-discoverer of the molecular shape of DNA, and Dr. Leslie Orgel, a British chemist also of the Salk Institute.
But Dr. Orgel said in an interview that it was not necessary to invoke the idea of panspermia to account for the very early appearance of microorganisms on earth suggested by the recent discoveries in Australia. Supended Animation
"I'm not really surprised by Dr. Schopf's discovery, nor am I convinced that it necessarily takes a long time for such organisms to develop after life begins," Dr. Orgel said. "Also, I've never insisted that life began elsewhere. It was only a suggestion that might be considered."
Dr. Crick has suggested that intelligent extraterrestrial beings, knowing that life has great difficulty getting started from simple chemicals, might have deliberately seeded the galaxy with such durable organisms as spores or bacteria, capable of surviving eons of space travel in suspended animation.
Very old rock is difficult to find, Dr. Schopf said, because most of it is uplifted into mountain ranges and then eroded away over the ages. Any fossils it might have contained are destroyed by this process.
Rocks dating from the Archean eon, or Early Precambrian period, are extremely rare, and authenticated biological fossils were almost unknown until the recent discovery. But Dr. Schopf believes that the near total absence of life in the fossil record during the earth's early period does not mean that there was no life, but merely that most fossils were destroyed by geological processes.