Evidence for Life on Earth More Than 3850 Million Years Ago

Heinrich D. Holland

The author [HN11] is in the Department of Earth and Planetary Sciences, Harvard University, [HN12]Cambridge, MA 02138, USA. E-mail: solomon@eps.harvard.edu

 

Some years ago, the remains of what were almost certainly microorganisms were discovered in the 3450-million-year-old Warrawoona sedimentary rocks of northwestern Australia (1). [HN1], [HN2]The evidence for life in these rocks rests on the morphology of the carbonaceous remains and the isotopic composition of the contained carbon. In the late 1970s, the presence of even earlier life on Earth was proposed on the basis of carbonaceous residues in 3800-million-year-old rocks at Isua in western Greenland (2). [HN3]Unfortunately, the rocks at Isua have been deformed at such high temperatures and pressures that the original morphology of the contained organic matter was lost. To complicate matters further, the isotopic composition of the bulk carbon probably changed during metamorphism. The evidence for life on Earth as early as 3800 million years ago has therefore been weak and equivocal. New isotopic data for carbon in these and in even older rocks have now been published by Mojzsis et al. (3): Their results elevate the hints of a very early origin of life on Earth to a solid suggestion.

Mojzsis et al. (3) have found that tiny bits of elemental carbon trapped in apatite [Ca5(PO4)3(OH,F)] in 3800- million-year-old and older rocks of western Greenland have isotopic compositions that span much of the range found in living and ancient microorganisms. [HN4]Their carbonaceous inclusions typically have a volume of about 10 microm3 and contain around 20 pg of carbon. The authors have determined the isotopic composition of the carbon using a new ion microprobe technique. New techniques are often fraught with uncertainty, but these results are probably reliable. [HN5]The measurements of delta13C in organic matter enclosed in apatite from somewhat younger (about 3250 million years old) and only slightly metamorphosed sediments from the Pilbara craton in western Australia fall within the normal delta13C range of organic carbon in most sedimentary rocks and agree with earlier measurements of delta13C in samples from the Warrawoona group (see column A in the figure).[HN6]


Written in stone. Isotopic composition of carbon in rocks that are over 3000 million years old. The lighter circles represent the data from Mojzsis et al. (3). The darker circles represent previous whole-rock measurements. Ma, million years ago. [Adapted from (3)]

The data (3) for delta13C in carbon from samples of a banded iron formation (BIF) at Isua are very similar to those of the younger Pilbara sediments. The values are significantly more negative than earlier measurements (marked 2 and 3 in column B of the figure) made on bulk carbon samples by standard techniques. The similarity of the new delta13C measurements and those of most younger sediments suggests a similar origin for the original organic matter. The difference between the old and the new measurements of Isua rocks is consistent with the view that the isotopic composition of their bulk carbon was altered during metamorphism by equilibration with the carbon in coexisting carbonates, whereas the original delta13C values of the bits of organic carbon analyzed by Mojzsis et al. were preserved by their protective apatite envelopes.

The isotopic composition of carbon in even older samples of a BIF on Akilia Island, also in western Greenland (column 3 in the figure), is more variable and spans much of the range of delta13C values found in organic matter of all ages. [HN7] The very negative delta13C values in their data are typical of organic matter produced by methanotrophs, organisms that use methane as their .

Do these data prove that life existed on Earth more than 3850 million years ago? Probably not. Other mechanisms can be proposed to account for the isotopic data. The isotopes of carbon are fractionated during the formation of organic molecules in electric discharges (4). The degree of fractionation observed in most of the laboratory experiments is smaller than that found by Mojzsis et al., but larger fractionations in nature cannot be ruled out. Light carbon from meteorite infalls is also possible. Isotopic fractionation during the metamorphism of organic matter has been explored by Mojzsis et al. and turns out to be very unlikely. The most reasonable interpretation of the data is surely that advanced by the authors: life existed on Earth more than 3850 million years ago.

In one sense this is not surprising. The Earth is known to have formed about 4550 million years ago--some 750 million years before the formation of the rocks at Isua, and probably not much less than 700 million years before the formation of the Akilia BIF. That is about the amount of time that has elapsed since the evolution of the first animals. On the other hand, the earliest history of the Earth was very violent. Large meteorite impacts, such as the impact that marked the Cretaceous-Tertiary boundary 65 million years ago must have been common, and their rate of infall may not have subsided until about 3800 million years ago. [HN8] The presence of life on Earth more than 3850 million years ago suggests that the destructive effects of bolide impacts had decreased dramatically before that time, or that life was invented more than once, or that our distant ancestors were a truly hardy lot. DNA sequencing has suggested that the earliest organisms were thermophilic, allowing survival in oceans that were heated by volcanoes, hot springs, and bolide impacts. [HN9]

The geologic evidence in the sedimentary rocks at Isua points to a planet that by 3800 million years ago had already settled down to its present, somewhat humdrum existence. By this point, its exuberant youth seems to have been past, and the relatively stable conditions required for the continuity of life had been established. This observation is surely consistent with the newly reported isotopic data (3), which strengthen the case for the proposition that life began long before 3500 million years ago. It will require the discovery of less highly metamorphosed rocks of Isua age to prove that the proposition is correct. Ultimately, we may even discover rocks that precede the origin of life on Earth and thus define its advent.

 

References

  1. See, for instance, J. W. Schopf, Science 260, 640 (1993).
  2. M. Schidlowski, P. W. U. Appel, R. Eichmann, C. E. Junge, Geochim. Cosmochim. Acta 43, 189 (1979).
  3. S. J. Mojzsis et al., Nature 384, 55 (1996). [Medline]
  4. S. Chang, D. DesMarais, R. Mack, S. L. Miller, G. E. Strathearn, in Earth's Earliest Biosphere, J. W. Schopf, Ed. (Princeton Univ. Press, Princeton, NJ, 1983), chap. 4, pp. 53-92. [HN10]

Volume 275, Number 5296 Issue of 3 Jan 1997, pp. 38 - 39
©1997 by The American Association for the Advancement of Science.