Science informs us that microbes were the first inhabitants of Earth. These are single-celled organisms which we commonly call bacteria, fungi and protozoa. Microbes are still found everywhere today, in boiling hot thermal springs, as well as deep below the surface of the Antarctic, and even high in the atmosphere.

Microbes decompose the waste products of other living things, creating nutrients. They are also used to make beer, bread, and yogurt. Did I hear you say...yak!?

Earth formed about 4.5 billion years ago. No one knows for a fact when or how life began. The final, most important events leading to the beginning of life are possibly the least understood chapters of the story.

Yet some things are pretty well agreed upon. Early Earth was dominated by volcanoes, a gray, lifeless ocean and a turbulent atmosphere. Intense chemical activity occurred in heavy clouds; these were fed by volcanoes and penetrated both by lightning discharges and also solar radiation.

The oceans received organic matter from land and atmosphere, as well as from the falling in of meteorites and comets. Substances as H2O, CO2, methane and hydrogen cyanide formed key molecules as sugars, amino acids and nucleotides. Such molecules are the building blocks of proteins and nucleic acids, compounds ubiquitous to all living organisms.

An early triumph was the development of critical RNA and DNA molecules, which directed biological processes and preserved life's 'operating instructions' for future generations down the ladder of time. Yet the origin of life was triggered not only by special molecules as RNA or DNA, but also by the chemical and physical properties of Earth's then very primitive environments.

Most of life's history involved the biochemical evolution of single-celled micro-organisms. We find individual fossilized microbes in rocks dating to over 3 billion years.

A decade ago, in an interesting finding - cover story of the Nov. 7, 1998 issue of the journal Nature - scientists reported that life on Earth began at least 3.85 billion years ago. The international team was composed of scientists from UC San Diego's Scripps Institution of Oceanography, UCLA's Department of Earth and Space Sciences, the Australian National University and England's Oxford Brookes University. They present evidence that pushes back the emergence of life on Earth by some 400 million years.

The evidence comes from a rock formation discovered on Akilia Island in southern West Greenland that is at least 3.85 billion years old. The research -- funded primarily by the National Science Foundation and NASA -- has provocative implications.

"Our evidence establishes beyond reasonable doubt that life emerged on Earth at least 3.85 billion years ago, and this is not the end of the story," said Stephen J. Mojzsis, a graduate student in geochemistry at Scripps and the lead author of the article. "We may well find that life existed even earlier".

"We look in rocks like this for chemical suggestions and isotopic evidence, and we found both," said T. Mark Harrison, professor of geochemistry at UCLA and director of UCLA's W. M. Keck Foundation Center for Isotope Geochemistry. "It would be wonderful to see a head and toes, and while we don't have those, we have found very strong isotopic evidence for ancient life".

"But in the cases of Earth's most ancient rocks and minerals, we are actually better off relying on this type of isotopic evidence -- chemo fossils -- rather than on the shape of life-like objects with which nature has often been deceiving the unwary," said Gustaf Arrhenius, professor of oceanography at UC San Diego and principal investigator for the research project.

The carbon inclusions in the rock were analyzed with UCLA's high-resolution CAMECA IMS 1270 ion microprobe -- an instrument that enables scientists to learn the exact composition of samples - which Mojzsis described as the "world's best instrument" for this research; (meanwhile a newer version, the IMS 1280, has been released to the markets). The microprobe shoots a beam of ions -- charged atoms -- at a sample, releasing from the sample its own ions that are analyzed in a mass spectrometer. Scientists can aim the beam of ions at specific microscopic areas of a sample and analyze them.

                                  
                                                                                   - www.cameca.fr -

The team of scientists, Mojzsis; Arrhenius, who is his research adviser; Harrison; Kevin McKeegan, a researcher in UCLA's Department of Earth and Space Sciences; Allen Nutman, a research fellow at the Australian National University; and Clark Friend, a geologist at Oxford Brookes University, presents the following evidence for the ancient life:

Most importantly, a high ratio of one form -- an isotope -- of carbon to another, which provides a "signature of life", Mojzsis said. The carbon aggregates in the rock have a ratio of about 100 to one of 12C (the most common isotope form of carbon, containing six protons and six neutrons) to 13C (a rarer isotopic form of carbon, containing six protons and seven neutrons). "The light carbon, 12C, is more than three percent more abundant than scientists would expect to find if life were not present, and three percent is, in this case, a very large amount," Arrhenius said.

The inclusion of the carbon in a phosphate mineral called apatite, which is also the material of which bones and teeth are made. Apatite is often formed by microorganics, but it can also be formed inorganically. The association of the carbon with the apatite is "suggestive, and not surprising, but does not in itself establish life," Arrhenius said.

The form of life discovered was probably a simple micro-organism, although its actual shape or nature cannot be ascertained, Mojzsis said, because heat and pressure over time have destroyed any original physical structure of the organisms.

Harrison, who directs UCLA's ion microprobe, said of the research, "This was a scientific problem that was waiting for a new generation microprobe of this resolution. The individual samples are very small, and no other instrument would have been sensitive enough to reveal precisely the isotopic composition and location of the carbon inclusions in the rock".

It is unknown when life first appeared on Earth, which is approximately 4.5 billion years old. The previous earliest evidence for life was presented by UCLA paleobiologist J. William Schopf, who showed that on the basis of bacteria-like fossils, primitive life, much like modern "pond scum," existed on Earth 3.46 billion years ago. "The evolution of lifeless matter into primitive life forms, and their organization into the complex structure of cells like those found by Schopf, represent an enormous development in the earliest history before the deposition of the Akilia sediments," Arrhenius said.

The residues of ancient life that the scientists have discovered existed prior to the end of the "late heavy bombardment" of the Moon by large objects, which ended approximately 3.8 billion years ago, Harrison said. The implication, he added, is that the often assumed simultaneous bombardment of Earth did not lead to the extinction of life.

This research shows that life on Earth began during the first approximately 700 million years after the formation of the planet, placing an upper limit on the time needed for the creation of life on Earth, or on the time period available for it to arrive here from elsewhere, the scientists said.

"Life is tenacious, and it completely permeates the surface layer of the planet," Mojzsis said. "We find life beneath the deepest ocean, on the highest mountain, in the driest desert and the coldest glacier, and deep down in the crustal rocks and sediments. Not knowing what conditions are needed for the emergence of life, it is only possible to speculate about its existence elsewhere in the universe. An important contribution to the solution of this problem could come from exploration of the surface of Mars for traces thereof extinct life".

An equally interesting question, the scientists agreed, that is currently studied in laboratories on Earth is how life originally could have arisen from lifeless molecules, and evolved into the already sophisticated isotope fractioning life forms recorded in the Akilia rocks.

Sources: www.nature.com,  www.space.com, www.xtronix.ch