When Did Life Arise on Earth

Life appears to have had little trouble in emerging on Earth. We know our planet formed about 4.55 billion years ago. A maximum of 700 million years after the formation of Earth — 3.85 billion years ago — it seems that life had evolved. We believe this to be the case because certain sedimentary rocks in Isua, Greenland — rocks that are among the oldest on this planet — contain isotopes of carbon in a ratio that is a sign of biological processes.

(The interpretation of these measurements is not without controversy. It may turn out that non-biological processes can generate a similar isotopic ratio of carbon. Nevertheless, many biologists accept that life was in existence at this time.) Since these are among the earliest known rocks, we can say there is little direct geological evidence for there ever being a time when life was absent from Earth! The earliest fossils are not much younger than the Isua rocks; stromatolites — mounds built up of layers of cyanobacteria and trapped sediment — are preserved as fossils in the Warrawoona Group in Western Australia. These stromatolites are 3.5 billion years old.

The haste with which life arose is almost too quick for comfort. The timespan mentioned above for the emergence of life, namely 700 million years, is an upper limit: that time span is squeezed from both ends. On the one hand, there was presumably some evolutionary process leading to the life-forms we find in the Greenland rocks; certainly the cyanobacte-ria of the Warrawoona Group had a biochemistry as sophisticated as other forms of life. In other words, if we found older rocks we might well find evidence for life in those rocks — perhaps simpler forms of life, but life nonetheless. Thus, life almost certainly emerged before Earth was 700 million years old. On the other hand, life could not have survived conditions on the very early Earth. (The initial period after formation of Earth, some 4.55 to 3.9 billion years ago, is called the Hadean era. The Isua rocks were laid down in the Early Archean era, which runs from 3.9 to 2.9 billion years ago.) As discussed on page 186, the early part of the Hadean era saw Earth peppered with impacts from large bodies. It is difficult to comprehend the violence of the literally Earth-shattering impact that gouged out the material that became our Moon. Certainly the impact would have sterilized the Hadean Earth: if any form of life was in existence before the impact, it could not have survived. So the period of 700 million years postulated for the emergence of life is an upper limit: the actual period was probably less than this.

Although several hundred million years may seem to offer plenty of time for life to evolve, it is worth remembering that the gap between life and non-life is huge, and that evolution can be a slow process. As the biologist Lynn Margulis famously put it: "The gap between non-life and a bacterium is much greater than the gap between a bacterium and man." And yet this gap was bridged relatively quickly. Some scientists find it difficult to accept that life could have begun so early on Earth without help, and have resorted to the panspermia hypothesis (see page 44).

If life indeed came to Earth through space, then there are implications to be considered for the Fermi paradox. The implications, however, depend upon exactly where the seeds of life came from. If life traveled through interstellar space and seeded our planet, then presumably there are count less numbers of planets in the Galaxy that were similarly seeded. Life will be everywhere. On the other hand, some astrobiologists have suggested that life originated on Mars — where conditions may have been more conducive to the development of life — and was transported to Earth on rocks that were ejected into space following impact events. As Mars lost its water, life died there; as conditions on Earth became more settled, life flourished here. If this is what happened, then life may be scarce even if life itself forms easily. It could be that two planets are required for life to prosper: a small planet on which life can originate, a nearby, more massive planet that can provide a long-term home for life, and meteorite impacts generating sufficient ejecta to transport life from one planet to the other. Such a combination of circumstances could be highly improbable.

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