Scientists looking for the oldest signs of earthly life have to take their evidence with considerable salt. They can easily fool themselves into seeing fossils or biological chemicals where no life would have been found.
This seems to have happened with the oldesst rocks known on Earth -- the Isua formation in Greenland. Dating back some 3.8 billion years, they contain microscopic forms and organic chemicals that have been taken for fossil microbes and their byproducts. Since Earth itself is "only" 4.6 billion years old, this would point to life having evolved rather quickly, geologically speaking.
Unfortunately, subsequent study by two research teams has called this intriguing finding into question.The story illustrates the ennui of trying to trace earthly life back to its origins.
The implication of 3.8 billion-year-old life has developed over the past few years from several directions. For example, an international research effort was launched to study Isua rocks brought back from a 1977 field trip. Scientists associated with the Max Planck Institute at Mainz, West Germany, who have been involved in this published findings that included the suggestion that the rock contains evidence of photosynthesis. This was supposedly reflected in the type of carbon found. Also, one of the team -- Jurgen Hahn -- reported finding breakdown products of chlorophyll.
In a companion study, Dieter Pflug of the University of Giessen, also i Germany, believed that he had found fossils of single-cell organisms. These would be the oldest fossils known.
These studies were carefully conducted. But they also depend on interpretation of data which is rather tricky. The "fossils," for example, are identified because they look like microbes -- not because there are unambiguous signs that they are indeed biological remains. Likewise, the chemicals found may be organic, but that is no guarantee that they were produced biologically. And even if the remains are biological, it is hard to prove they were not introduced into the rock at a relatively recent stage.
Such are the traps for paleobiologists. And it is into such traps that the Mainz group seems to have fallen, according to a pair of papers recently published in Nature.
In one of these, an international team of nine scientists points out that Pflug's "yeastlike microfossils" are "indistinguishable from . . . microstructures which are demonstrably inorganic. . . ." In the other paper, Bartholomew Nagy and four colleagues at the University of Arizona explain why it is unlikely that any biological chemicals would have survived the known geological transformations of the Isua rocks. Hence they think any such chemicals are later intrusions.
The issue will undoubtedly be debated. Meanwhile, though we're left with the earlier, and younger, estimate of the dawn of earthly life -- a mere 3.3 billion years or so ago.