What a Wonderful World by Marcus Chown Read Free Book Online Page B

that which we might expect from the laws of chance,’ wrote American biologist Gilbert Newton Lewis. 13
    But evolution by natural selection has its limits. The only organisms that can arise are those that are the result of a long string of advantageous changes. ‘Evolution walks backwards into the future,’ says British biologist Steve Jones. ‘It doesn’t know what’s coming.’ 14 This has led some people to claim that Darwin’s theory cannot explain the existence of complex organs such as the eye, which consists of multiple components. Until all components are in place – a lens, a light-detecting surface, and so on –goes the argument, no advantage is conferred on an organism. What use is 50 per cent of an eye? Or 5 per cent of one?
    However, it turns out that all the steps along the road to the eye were indeed advantageous. Examples of primitive eyes can be seen throughout the animal kingdom. Some creatures have only a patch of light-sensitive cells for sensing which way is up and which down. Others, like the pit viper, have light-sensitive – actually, heat-sensitive – cells at the bottom of a pit in their skin, so their ‘sight’ has a directional capability. From this, it is a short step to close over the pit with a transparent protein, creating a lens that can focus an image of an object.
    In addition to having no foresight, evolution by natural selection does not necessarily result in more complex forms. It
can
, but it does not always do so. After the advent of the first cell, there really was nowhere to go but
up
in terms of size and complexity. But, as soon as larger creatures evolved, it was possible to evolve back down to simpler forms. This can be seen in the case of parasites, which live off their more complex hosts.
    Darwin’s theory of evolution by natural selection – Dawkins’s ‘greatest idea in the history of science ’ – has passed every test. ‘It could so easily be disproved if just a single fossil turned up in the wrong date order,’ wrote Dawkins. 15 All it would take would be the discovery of a rabbit in the pre-Cambrian period 500 million years ago. As yet, this has not happened.
    Notes
    1 Useful traits are not only those that boost a creature ’s chance of surviving long enough to reproduce but also those that boost a creature ’s chance of getting the
opportunity
to reproduce if it survives that long. Such sexually selected traits include the peacock’s tail – which makes a male attractive to a female – and a stag’s antlers – which enables a male to out-compete other males for a mate.
    2 Alfred Russel Wallace exempted humans from the process of naturalselection. He therefore avoided the controversy that surrounded Charles Darwin – and also the fame. Wallace ’s collected works – books, articles, manuscripts and illustrations – can be found at http://wallace-online.org.
    3 The complete works of Charles Darwin can be found online at http://darwin-online.org.uk.
    4 Actually, our Milky Way Galaxy turns out not to be at the centre of things but merely one among 100 billion or so others in our Universe. And there is a growing suspicion that our Universe itself is not special but merely one among countless others in a multiverse. So, seen in this context, Darwin is merely one of many scientists who have applied the Copernican principle, moving humans remorselessly from the centre of the world and revealing their insignificance in an indifferent, bewilderingly huge, and possibly infinite, cosmos. See Chapter 21, ‘The day without a yesterday: Cosmology’.
    5 See Chapter 1, ‘I am a galaxy: Cells’.
    6 The copying of DNA is made possible by a remarkable circumstance .
A
always pairs with
T
, and
G
with
C
. So, if a cell’s double helix of DNA is split down the middle, it forms two complementary strands.
A
s floating about in solution automatically lock like jigsaw pieces to exposed
T
s;
T
s mesh with
A
s;
G
s with
C
s; and
C
s with
G
s. The result