Monday, November 02, 2015
The Oldest Evidence of Life on Earth
by Paul Braterman
It looks as if life on Earth just got older, and probably easier. Tiny scraps of carbon have been found inside 4.1 billion year old zircons, and examination shows that this carbon is most probably the result of biological activity. This beats the previous age record by 300 million years, and brings the known age of life on Earth that much closer to the age of Earth itself. The implication is that life can originate fairly quickly (on the geological timescale) when the conditions are right, increasing the probability that it will have originated many times at different places in our Universe.
The Solar System, it is now thought, formed when the shockwave from a nearby supernova explosion triggered a local increase in density in the interstellar gas cloud. This cloud was roughly three quarters hydrogen and one quarter helium, all left over from the Big Bang some 9 billion years earlier. It had already been seeded with heavier elements produced by red giant stars, to which was now added debris from the supernova, including both long-lived and short-lived radioactive elements. Once the cloud had achieved a high enough local density, it was bound to fall inwards under its own gravity, heating up as it did so. The central region of the cloud would eventually become hot enough and dense enough to allow the fusion of hydrogen to helium. A star was born.
The heavy elements (and in this context “heavy” means anything heavier than hydrogen and helium) in the dust cloud surrounding the nascent Sun gave rise to the rocky cores hidden within the outer giants Jupiter, Saturn, Neptune and Uranus, of the outer reaches of the Solar System, and to the rocky inner planets, Mercury, Venus, Mars, and, of course, to Earth and everything upon it. We are stardust.
The asteroids are made out of material that was never able to come together to form a planet, because of the competing gravitational pull of Jupiter. Asteroids are continually bumping into each other, scattering fragments, and some of these fragments fall to earth as meteorites. The Hubble Telescope has given usimages of star and planet formation in progress. Such is our modern creation myth, magnificent in scale, and rooted in reality.
The oldest solid objects in the Solar System are calcium-aluminium rich grains, the most refractory of all the materials to condense out of the gas cloud. These are now known from a refined form of uranium-lead dating  to have formed as much as 4,568.2 million years ago, give or take a very few hundred thousand years either way, and that is now the accepted best estimate for the Solar System’s age. A remarkable feat, to fix this to within around 1% of 1%. As time went by, and the outer regions of the gas cloud radiated away their energy, more materials condensed out, and the grains grew and stuck together by contact and eventually by their own gravity. Thus we went from grains to pebbles to larger objects to planetesimals and eventually to the planets as we know them. The final stages were marked by increasingly violent collisions, culminating in the collision between the proto-Earth and a Mars-size object that gave rise to the present Earth-Moon system, and rounded off by what has been called the Late Heavy Bombardment .
The energy of the collisiodensens will have caused melting, even before the formation of full-scale planetesimals, and the separation of the molten bodies into a metal-rich (mainly iron) core, and a less , oxygen-rich outer mantle. It is Earth’s iron core that is responsible for its magnetic field, and this field in turn shields us from the constant bombardment of charged particles emanating from the Sun, which would otherwise have stripped away our atmosphere. Elements like platinum and gold (so-called siderophiles, or iron-lovers) concentrated in the core, which is one reason why they are so rare at the surface, while elements such as oxygen, calcium, magnesium, aluminium and silicon are lithophiles, or rock-lovers, and concentrate in the mantle. Fortunately, the highest melting point rocks, which are thus the first to solidify, are less dense than average, which is why Earth has a solid crust floating on the surface of the mantle. The precious metals are all much stronger siderophiles than iron itself, which forms a strong bond with oxygen and is one of the most common elements in the crust and mantle, as well as being the main constituent of the core. The Late Heavy Bombardment explains the craters on Mercury, the Moon, and Mars. No such craters survive on Earth, but that is because weathering and plate tectonics have completely reworked the surface.
We can learn a lot about the history of these processes from the distribution of the different elements, and even of individual isotopes, especially radioactive isotopes and their decay products. For example, hafnium-182 is radioactive, with a half life of slightly under 9 million years, decaying to tungsten-182. Hafnium is a lithophile, and tungsten a siderophile. So if core formation is slow on this timescale, most of the hafnium-182 from the supernova debris will have had time to decay to tungsten, and will vanish into the core. But if core formation is relatively fast, the hafnium-182 will remain in the rocky phase, where the tungsten-182 derived from it will end up stranded.
We can also sometimes learn about how a material was formed by looking at the ratio of different non-radioactive isotopes. Almost all elements occur as more than one isotope, with the same number of protons and electrons, but different numbers of neutrons. You may well have been told at school that isotopes, despite their have different masses, have identical chemistry, but this is not quite true. Generally speaking, because of quantum mechanical effects , different isotopes have very slightly different chemistries, and small deviations in their relative abundance provides clues to a sample’s history.
Using many detailed arguments of this kind, we come up with the following sequence:
- Beginning of solar system, 4,568 million years ago (see above)
- Collisions between planetary embryos, and partial melting of resulting meteorites, within a very few million years of that beginning
- Accretion of Earth under way within 10 million years of beginning
- Earth-Moon system formed, between 30 and 100 million years from the beginning. Formation of the Earth’s liquid core would be complete at this stage, although the formation of the solid inner core is remarkably recent by comparison (around 1,000 to 1,500 million years ago)
- Oldest rocks on moon 4,460 million years old, (dating Moon’s oldest crust to within a very few tens of millions of years after its formation)
- Oldest rocks on Earth, 3960 million years old, with evidence for an older (4000 to 4200 year old) component
- Late Heavy Bombardment, around 3,900 million years ago, as estimated by dating craters on the Moon.
It was at one time assumed that the Late Heavy Bombardment would have heated the Earth’s surface sufficiently to destroy any life forms in existence at that time. But careful estimates of the total heating effect show that this is not the case, even at the surface, while bacteria obtaining their energy from reactions involving minerals have been found 2.8 kilometers below the surface.
The Jack Hills of Western Australia are of enormous interest to geologists. The rocks that they are made of are thought to have been originally laid down some 3,600 million years ago, as deposits from river deltas, although they have undergone many episodes of transformation since then. They are of special interest because the delta deposits contained zircons that were already, at that time, hundreds of millions of years old; tough grains of impure zirconium silicate from the already ancient mountains, eroded out by the streams that fed the deltas, and transported and buried there unchanged. These have inspired a truly heroic effort from geologists; one paper, in its title, refers to “The first 100,000 grains.” Two separate research groups have reported that the oldest zircons found there, dating back to 4,400 million and 4,300 million years ago, show evidence for the presence on the planet of liquid water , which is generally regarded as a necessary condition for the emergence of life.
Necessary, but not sufficient.
We turn now to the oldest evidence for life on earth.
Hard fossils of complex organisms first appear in abundance around 545 million years ago, although we can stretch this back to 600 million years if we include fossilised traces, such as burrows (here, Ch. 7). If we want to go back much further, we will be relying on evidence from single-celled organisms, which is always less clear-cut and more open to alternative explanations. However, such organisms can form mats, with a characteristic texture that develops from horizontal layers of dead organisms, with trapped soil particles between them. This leads to the development of what are known as stromatolites, domed multi-layered structures that persist to the present day. Modern stromatolites, at least, are quite complex communities of cyanobacteria, single celled organisms capable of photosynthesis, with different kinds of bacteria, using different wavelengths of light, found at successive levels. Stromatolites are found throughout the fossil record; they were at their most abundant some 1,500 million years ago, but are now found mainly in highly saline lagoons, where grazing creatures, which disturb their formation, cannot survive. The oldest fossil stromatolites are found embedded in 3,430 million year old chert (silica rock), and if we make the reasonable assumption that continuity of form represents continuity of kind of organism, it follows that diverse communities of photosynthesising bacteria were already in existence at that time.
There are claims of microfossils of chains of bacteria, going back to 3,600 million years ago, but these are little more than dark smudges embedded in chert, and their interpretation remains controversial. Moreover, rocks of this age or older have all undergone considerable change, having been subjected at various times to great pressure and high temperatures. To go back further, we have to resort to more indirect kinds of evidence.
Carbon occurs on Earth as a mixture of two main isotopes, carbon-12 (99%) and carbon-13 (1%). There are also traces of carbon-14, used in radiocarbon dating, but this has a half life of only some 5700 years and apart from contamination is effectively absent from materials over a million years old . It has been known since 1939 that the isotopic composition of carbon in plants is different from that found in the carbon dioxide from which it is derived; plant carbon, and materials derived from it, are “light”, meaning that they have a measurably smaller proportion of carbon-13. This is as expected  from quantum mechanics, which predicts that common dioxide containing carbon-12 will be slightly more chemically reactive than that containing carbon-13. The excess of carbon-12 is, of course, inherited by all materials derived from plants, such as animals (which eat them), and fossil fuels. Indeed, one of the many ways in which we know that the recent unprecedented rapid increase in atmospheric carbon dioxide is the result of our burning fossil fuels, is the increasing proportion of carbon-12 in atmospheric carbon dioxide over time.
In 1995, I had the privilege of visiting the laboratories of Gustaf Arrhenius at the Scripps Institution of Oceanography, La Jolla. There I met a Ph.D. student, Steve Mojzsis, who has gone on to pursue a distinguished career in isotope geochemistry. Steve is now Professor at the University of Colorado at Boulder, and his research group was responsible for several of the findings described above. As his Ph.D. problem, Steve was examining 3,800 million year old sediments from Greenland, which were known to contain carbon slightly, but perhaps not conclusively, lighter than expected. Within these rocks, he found grains of hydroxyapatite, which is a very tough form of calcium phosphate, essentially the same as the material your teeth are made of. And within these grains were tiny granules of carbon.
What happened next was made possible by advances in scientific instrumentation, and specifically in the development of what is known as iron microprobe mass spectrometry (more fully, Sensitive High Resolution Ion Microprobe or SHRIMP). This is just what the name implies. A beam of charged particles (ions) is accelerated and focused, and used to drill away at an area of the sample a hair’s breadth across. The fragments blasted out by this process are then fed into a mass spectrometer, which sorts out the different isotopes. When the carbon granules were examined in this way, they were found to be within the range expected for organic material arising by photosynthesis. So these granules were biological in origin, and the earlier inconclusive results were the result of averaging out organic and inorganic material.
Science does not provide proofs, at least not in the sense that mathematics provides proofs, and there are alternative non-biological routes to light carbon. But these involve reactive metals that would not have been present in the crust after core formation, and in any case such processes would not account for the segregation of the light carbon within granules. And so, while scientific conclusions are always in principle subject to being overturned by new evidence, my own view is that it would be unreasonable to deny this evidence for life 3,800 million years before the present.
Steve’s record stood for 20 years, but has just been spectacularly broken, as a result of the zircon screening that I mentioned earlier. Some of the oldest zircons contain flecks of carbon, visible under the microscope. One of these was selected for special examination, cut open, and the carbon examined. Radiometric dating of the freshly cut zircon surface gave a date of 4,100 million years old, while the carbon itself turned out to be light, in the range expected for what had once been living material, with the carbon having been derived from carbon dioxide by photosynthesis. Thus we can now say, with a surprising degree of confidence, that there was life on Earth, and indeed life capable of carrying out the complicated sequence of reactions necessary for photosynthesis, 4,100 million years ago.
So what does this tell us? Are we all descended from the life forms in existence at that time? Almost certainly yes. The alternative would be a far more complicated story, with life having arisen more than once. It follows that the life from which we are all descended was present on Earth within 350 million years of the formation of the Earth-Moon system, and within an even shorter time after Earth had developed a solid crust, cool enough for liquid water (a prerequisite of our form of life).
In 1981, Francis Crick wrote that “we can only say that we cannot decide whether the origin of life on earth was an extremely unlikely event or almost a certainty - or any possibility in between these two extremes.” Now, at last, we can go beyond this. If the origin of life was unlikely, then life originating so early would be even more unlikely. So while it may be putting it too strongly to say that its emergence was “almost a certainty”, we can say that it was certainly a reasonable possibility. And if it was a reasonable possibility here on Earth, then it must equally be a reasonable possibility on all the Earth-like planets we have discovered, whose number grows almost daily.
To quote Steve’s comment on these discoveries, “This is what transformative science is all about. If life is responsible for these signatures, it arrives fast and early.”
1] Technically speaking, lead-lead dating. This depends on the ratio of lead-206 (formed by decay of uranium-238) to lead-207 (formed from uranium-235), with non-radiogenic lead-204 as a measure of lead from other sources. The calculation depends on the known difference in half life between the parent uranium isotopes. We know that these half lives must have been constant, since they are not free variables but consequences of the more fundamental constants of nature, and had these been different then the meteorites would not have formed as they did in the first place.
2] One problem with this scenario is the extreme similarity in composition between Earth and Moon rocks, difficult to explain if they are derived from two separate parent bodies. See, however, here.
3] As a consequence of the uncertainty principle, all materials store an unremovable amount of what is called “zero point vibrational energy”, and the amount of this energy is proportional to vibrational frequency. Lighter isotopes are therefore associated with higher zero point energies, leading in general to slightly higher chemical reactivity.
4] The amount of the minor isotope oxygen-18 present in these samples is different from the bulk of the mantle from which they crystallised, and indicative of mantle formed from the remelting of crust that had exchanged oxygen-18 with liquid water.
5] There is a steady trickle of claims from Young Earth creationists to have detected carbon-14 in dinosaur bones, diamonds, and coal. The first two of these are explained by contamination, while the more interesting case of coal is associated with nuclear reactions involving other radioactive atoms trapped within the material.
General references hyperlinked in the usual way. Selected more technical references (some behind paywall but all with open abstracts): Potentially biogenic carbon preserved in a 4.1 Byo Zircon here; Solar system age here; Earth’s accretion here, here, and here; Moon formation here; late origin of earth’s inner core here; zircon mass screening here, Earth’s oldest surviving crust here, here and here; 4.4 Byo zircon here, and the existence of water on earth when oldest zircons formed, here and here; Habitability of Hadean Earth here; 3.43 Byo stromatolites here; Biological carbon isotope effect here; previous oldest evidence for life here. Image of zircon with granules via ibtimes. Stromatolite image from Britannica.
Posted by Paul Braterman at 12:25 AM | Permalink