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Runner Up: Original mysteries

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Science  20 Dec 1996:
Vol. 274, Issue 5295, pp. 1988
DOI: 10.1126/science.274.5295.1988a

How—and where—did life begin? That's one of humanity's oldest questions, and this year scientists in several fields suggested startling new answers, firing the public imagination with possible traces of ancient life on Mars and supporting one view of life's basic family tree with the genetic sequence of a bizarre microbe.

In August, a NASA-led team announced evidence of past life in an ancient martian meteorite, prompting a presidential pronouncement, grabbing front-page headlines, and refocusing NASA's space-exploration plans overnight. Each of the team's four lines of evidence—certain minerals, organic matter, chemical imbalances, and bacterialike structures—could be due to abiotic causes, but the group argued that taken together, the most likely explanation was life. Then in November, a British team suggested that another martian meteorite also held organic compounds indicative of ancient organisms.

But many scientists remain skeptical, noting that the organic matter could be the leftovers of abiotic chemical reactions or earthly contamination (see Technical Comments). More study could boost the case, if new life signs such as key amino acids or tiny structures turn up. But final answers may have to wait for rock samples from the Red Planet—which aren't due until after the turn of the century.

Back on Earth, isotopic clues in Greenland rocks suggested that life had appeared here by 3.8 billion years ago. And also this year, scientists presented genetic data showing that life falls into just three major domains, rather than the five kingdoms of classical textbooks. This trinitarian view was proposed in the 1970s, but this year's work all but clinched the case. The genetic sequence of a heat-loving microbe, Methanococcus jannaschii—a member of one of the big three, the Archaea—was strikingly different from sequences from the other two groups, bacteria and eukaryotes (which include all plants and animals). The data suggest that the Archaea can no longer be lumped with bacteria, and indeed are probably closer kin to eukaryotes. Researchers are now unraveling more genomes from primitive organisms, hoping to work backward to the very root of life on Earth.

ADDITIONAL READING

News Stories:

  • J. Kaiser, “2003 Is Earliest to Retrieve Samples,” Science, 23 August 1996, p. 1040.

  • R. A. Kerr, “Ancient Life on Mars?” Science, 16 August 1996, p. 864.

  • R. A. Kerr, “Martian Rocks Tell Divergent Stories,” Science, 8 November 1996, p. 918.

  • V. Morell, “Life's Last Domain,” Science, 23 August 1996, p. 1043.

Research Papers:

  • D. S. McKay et al., “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001,” Science, 16 August 1996, p. 924.

  • S. J. Mosjzsis et al., “Evidence for Life on Earth Before 3,800 Million Years Ago,” Nature 384, 55 (1996).

Link:

  • For more on Mars meteorites and links to other sites, see NASA's meteorite page.

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