The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

Cellular slime moulds are social amoebas. They literally blur the distinction between a social group of individuals and a single multicellular individual. In part of their life cycle, separate amoebas creep through the soil, feeding on bacteria and reproducing, as amoebas will, by dividing in two, feeding some more, then dividing again. Then, rather abruptly, the amoebas switch into ‘social mode’.

They converge on aggregation centres, from which chemical attractants radiate outwards. As more and more amoebas stream in on an attraction centre, the more attractive it becomes, because more of the beacon chemical is released. It is a bit like the way planets form from aggregating debris. The more debris accumulates in a given attraction centre, the more its gravitational attraction. So after a while, only a few attraction centres remain, and they become planets. Eventually the amoebas in each major attraction centre unite their bodies to form a single multicellular mass, which then elongates into a multicellular ‘slug’.

Terrestrial ourselves, our thoughts are preoccupied with the green and pleasant land. But about half of global photosynthesis occurs in the sea. The two environments pose quite different problems for a light harvester.

The Excavates

they include the nasty gut parasite Giardia and the sexually transmitted vaginal microbe Trichomonas.

Not all cause disease though.

Dinoflagellates belong to the Alveolates, the ‘A’ in SAR. They too are unicellular. A number of marine species give out tiny flashes of light when disturbed, which, it has been suggested, may attract fish to eat smaller animals that would otherwise prey on the dinoflagellates. Whatever the explanation, the effects can be spectacular at night, producing luminous blue surf and enveloping nocturnal swimmers in a haze of glowing water.

Within the alveolates lie the parasitic apicomplexans, photosynthesisers gone to the bad.

They include Plasmodium, the malaria parasite, which due to its light-harvesting past is sensitive to certain anti-plant compounds—a source of hope for new antimalarial drugs. Also to be found here is Toxoplasma, a mind-controlling parasite of rodents and cats, which is also commo...

But Stramenopiles have also independently discovered true multicellularity, in the form of the brown algae. These are the largest and most prominent of all seaweeds, with giant kelps reaching 100 metres in length.

The brown algae include the wracks of the genus Fucus, the various species of which segregate themselves in strata up the beach, each being best suited to a particular zone of the tide cycle.

The penultimate ‘kingdom’ comprises the taxonomically homeless creatures at the top of our star diagram.

Most important among those are the photosynthesising haptophytes, whose geometric armoured shield plates are seen under the microscope to be the main component of the White Cliffs of Dover and the huge beds of chalk throughout Western Europe, after which the Cretaceous Period is named.

The closest relatives of the land plants are another group of green algae, the freshwater charophytes, which implies that plants probably didn’t move directly from the sea onto the land but, like animals, went via freshwater. Fossils indicate this happened during the Ordovician, with arthropods probably following soon after.

THE REDWOOD’S TALE

the world’s tallest trees, Sequoia sempervirens, the Pacific coast redwoods,

The related species Sequoiadendron giganteum (see plate 50), found inland on the foothills of the Sierra Nevada range, is typically slightly shorter but more massive.

The largest single living creature in the world, the General Sherman tree, is a giganteum over 30 metres in circumference and over 80 metres tall, ...

Its age is not known for certain but the species is known to survive ...

Ring counting, in a more sophisticated form, has given rise to the elegant technique of dendrochronology, by which archaeologists working on a timescale of centuries can precisely date any wooden artefact.

Annual rings in a tree result from the unsurprising fact that a tree puts on more growth in some seasons than in others. But, by the same token, whether in summer or winter, trees grow more in a good year than in a poor year. Good years are two a penny, and so are bad years, so one tree ring is no good for identifying a particular year. But a sequence of years has a fingerprint pattern of wide and narrow rings, which labels that sequence in different trees over a wide area. Dendrochronologists compile catalogues of these labelled signature patterns. Then a fragment of wood, perhaps from a Viking longship buried in mud, can be dated by matching its ring pattern against previously collected libraries of signatures.

a newly felled tree, the outer ring represents the present. The past can be exactly reckoned by counting inwards. So absolute dates can be put upon ring pattern signatures in recent trees whose date of felling is recorded. By looking for overlaps—signature patterns near the core of a young tree that match the pattern in the outside layers of an older tree—we can put absolute dates on ring patterns in older trees too. By daisy-chaining the overlaps backwards, it is in principle possible to put absolute dates on very old wood indeed—in principle even from the Petrified Forest of Arizona, if only there were a continuous series of petrified intermediates—if only!

By this technique of overlapping jigsaws, libraries of fingerprint patterns can be built up and consulted to recognise wood that is older than the oldest tree we ever see alive. The changing thickness of tree rings can also, incidentally, be used not just for dating wood but for reconstructing year-to-year climate and ecological patterns dating from long before meteorological records were kept.

Dendrochronology is limited to the relatively recent time domains inhabited by archaeologists. But tree growth is not the only process that spurts and slows on an annual cycle, or on some other regular or even irregular cycle. Any such process can in principle be used for dating, aided by the same ingenious trick of daisy-chaining overlapped pat...

Sediments are laid down on the sea bottom at an uneven rate, and in stripes which we can think of...

Another example, which we encountered in the Prologue to the Sloth’s Tale, is palaeomagnetic dating. As we saw there, the Earth’s magnetic field reverses from time to time. What had been magnetic north suddenly becomes magnetic south for some thousands of years, then flips again. This has happened 282 times during the last 10 million years.

The magnetic North Pole in any case seldom coincides exactly with the true, geographic North Pole (around which the Earth spins). It wanders around the polar region

over the years. At present the magnetic North Pole is about 300 miles from the true North Pole, heading rapidly from Canada towards Russia.

A thousand years in geology’s sight is but an evening gone. The time spent ‘flipping’ is negligible compared to the time spent in the rough vicinity of either the true North or the true South Pole. Nature, as we saw earlier, keeps an automatic record of such events. In molten volcanic rock, certain minerals behave like little compass needles.

When the molten rock solidifies, these mineral needles constitute a ‘frozen’ record of the Earth’s magnetic field at the moment of solidification (by a rather different process, palaeomagnetism can be observed in sedimentary rock, too).

It’s like tree rings all over again, except that the stripes are not a year apart but of the order of a million years. Once again, patterns of stripes can be matched up with other patterns, and a continuous chronology of magnetic flips can be daisy-chained together.

Absolute dates can’t be calculated by counting stripes because, unlike tree rings, the stripes represent unequal durations.

Nevertheless, the same signature pattern of stripes can be picked up in different places. This means that if some other method of absolute dating is available for one of the places, magnetic stripe patterns, like the Parsons code for a melo...

Tree rings are good for dating recent relics to the nearest year. For older dates, with inevitably less fine pinpointing, we exploit the well-understood physics of radioactive decay.

All matter is made of atoms. There are more than 100 types of atoms, corresponding to the same number of elements. Examples of elements are iron, oxygen, calcium, chlorine, carbon, sodium and hydrogen. Most matter consists not of pure elements but of compounds: two or more atoms of various elements bonded together, as in calcium carbonate, sodium chloride, carbon monoxide. The binding of atoms into compounds is mediated by electrons, which are tiny particles orbiting (a metaphor to help us understand their real behaviour, which is much stranger) the central nucleus of each atom.

A nucleus is huge compared to an electron but tiny compared to an electron’s orbit. Your hand, consisting mostly of empty space, meets hard resistance when it strikes a block of iron, also consisting mostly of empty space, because forces associated with the atoms in the two solids interact in such a way as to prevent them passing through each other. Consequently iron and stone ...

It has long been understood that a compound can be separated into its component parts, and recombined to make the same or a different compound with the emission or consumption of energy. Such easy-come easy-go interactions between atoms constitute chemistry. But, un...

The modern view is more elegant. Gold atoms, copper atoms, hydrogen atoms and so on are just different arrangements of the same fundamental particles, just as horse genes, lettuce genes, human genes and bacterial genes have no essential ‘flavour’ of horse, lettuce, human or bacteria but are just different combinations of the same four DNA letters.

In the same way as chemical compounds have long been understood to be arrangements put together from a finite repertoire of 100 or so atoms, so each atomic nucleus turns out to be an arrangement of two fundamental particles, the protons and neutrons. A gold nucleus is not ‘made of gold’. Like all other nuclei, it is made of protons and neutrons. An iron nucleus differs from a gold nucleus, not because it is made of a qualitatively different kind of stuff called iron, but simply because it contains 26 protons and some neutrons, instead of gold’s 79 protons and some neutrons.

At the level of a single atom there is no ‘stuff’ that has the properties of gold or iron. There are just different combinations of protons, neutrons and electrons. Physicists go on to tell us that protons and neutrons are thems...

Protons and neutrons are almost the same size as each other, and much larger than electrons. Unlike a neutron, which is electrically neutral, each proton has one unit of electric charge (arbitrarily designated positive), which exactly balances the negative charge of one electron ‘in orbit’ around the nucleus. A proton can be transformed into a neu...

Conversely, a neutron can transform itself into a proton by expelling a unit of nega...

Such transformations are examples of nuclear reactions, as opposed t...

Chemical reactions leave the nucleus intact. Nuclear reactions change it. They usually involve much larger exchanges of energy than chemical reactions, which is why nuclear weapons are so much more devastating, weight...

Each element has a characteristic number of protons in its atomic nucleus, and the same number of electrons in ‘orbit’ around the nucleus: one for hydrogen, two for helium, six for carbon, 11 for sodium, 26 for iron, 82 for lead, 92 for uranium. It is this number, the so-called atomic number, which (acting via the electrons) largely determines an element’s chemical behaviour.

The neutrons have little effect on an element’s chemical properties, but they do affect its mass and they do affect its nuclear reactions.

A nucleus typically has roughly the same number of neutrons as protons, or a few more. Unlike the proton count, which is fixed for any given element, the neutron count varies. Normal carbon has six protons and six neutrons, giving a total ‘mass number’ of 12 (since the mass of electrons is negligible and a neutron weighs approximately the same as a proton). It is therefore called carbon 12. Carbon ...

‘versions’ of an element are calle...

The reason all three of these isotopes have the same name, carbon, is that they have the same atomic number, 6, and therefore all have the same chemical properties. If nuclear reactions had been discovered before chemical reaction...

In a few cases, isotopes are different enough to earn different names. Normal hydrogen has no neutrons. Hydrogen 2 (one proton and one neutron) is called deuterium. Hydrogen 3 (one proton and two neutrons) is called tritium. All behave chemically as hydrogen. For example, deuterium combines with oxygen to make a form o...