Underlying much of the fallacious demand for 'missing links' is a medieval myth, which occupied men's minds right up to the age of Darwin and stubbornly confused them after it. This is the myth of the Great Chain of Being, according to which everything in the universe sat on a ladder, with God at the top, then archangels, then various ranks of angels, then human beings, then animals, then plants, then down to stones and other inanimate creations. Given that this goes way back to a time when racism was second nature, I hardly need add that human beings were not all sitting on the same rung. Oh no. And of course males were a healthy rung above females of their kind (which was why I let myself get away with 'occupied men's minds' in the opening sentence of this section). But it was the alleged hierarchy within the animal kingdom that had the greatest capacity to muddy the waters when the idea of evolution burst upon the scene. It seemed natural to suppose that 'lower' animals evolved into 'higher' animals. And if this were so, we should expect to see 'links' between them, all the way up and down the 'ladder'. A ladder with lots of missing rungs lacks conviction. It is this image of the rungless ladder that lurks behind much of the scepticism about 'missing links'. But the entire ladder myth is deeply misconceived and un-evolutionary, as I shall now show.
So glibly do the phrases 'higher animals' and 'lower animals' trip off our tongues that it comes as a shock to realize that, far from effortlessly slotting into evolutionary thinking as one might suppose, they were - and are - deeply antithetical to it. We think we know that chimpanzees are higher animals and earthworms are lower, we think we've always known what that means, and we think evolution makes it even clearer. But it doesn't. It is by no means clear that it means anything at all. Or if it means anything, it means so many different things as to be misleading, even pernicious.
Here is a list of the more or less distinctly confusing things you might mean when you say, for example, that a monkey is 'higher' than an earthworm.
1 'Monkeys evolved from earthworms.' This is false, just as it is false that humans evolved from chimpanzees. Monkeys and earthworms share a common ancestor.
2 'The common ancestor of monkeys and earthworms was more like an earthworm than like a monkey.' Well, that potentially makes more sense. You can even use the word 'primitive' in a semiprecise way, if you define it as 'resembling ancestors', and it is obviously true that some modern animals are more primitive in this sense than others. What that exactly means, if you think about it, is that the more primitive of a pair of species has changed less since the common ancestor (all species, without exception, share a common ancestor if you go back far enough). If neither species has changed dramatically more than the other, the word 'primitive' should not be used in comparing them.
It's worth pausing here to make a related point. It is hard to measure degrees of resemblance. And there is in any case no necessary reason why the common ancestor of two modern animals should be more like one than the other. If you take two animals, say a herring and a squid, it is possible that one of them resembles the common ancestor more than the other, but it doesn't follow that this has to be the case. There has been an exactly equal amount of time for both to have diverged from the ancestor, so the prior expectation of an evolutionist might be, if anything, that no modern animal should be more primitive than any other. We might expect both of them to have changed to the same extent, but in different directions, since the time of the shared ancestor. This expectation, as it happens, is often violated (as in the case of monkey and earthworm), but there is no necessary reason why we should expect it to be. Moreover, the different parts of animals don't all have to evolve at the same rate. An animal might be primitive from the waist down but highly evolved from the waist up. Less facetiously, one of them might be more primitive in its nervous system, the other more primitive in its skeleton. Notice especially that 'primitive' in the sense of 'resembling ancestors' does not have to go with 'simple' (meaning less complex). A horse's foot is simpler than a human foot (it has only a single digit instead of five, for example), but the human foot is more primitive (the ancestor that we share with horses had five digits, as we do, so the horse has changed more). This leads us on to the next item in our list.
3 Monkeys are cleverer [or prettier, have larger genomes, more complicated body plans, etc. etc.] than earthworms.' This kind of zoological snobbery is a mess when you start trying to apply it scientifically. I mention it only because it is so readily confused with the other meanings, and the best way to sort out confusion is to expose it. You could imagine a large number of scales along which you might rank animals - not just the four scales I have mentioned. Animals that are high on one of these ladders may or may not be high on another. Mammals certainly have larger brains than salamanders, but they have smaller genomes than some salamanders.
4 Monkeys are more like humans than earthworms are.' This is undeniable for the particular example of monkeys and earthworms. But so what? Why should we choose humans as the standard against which we judge other organisms? An indignant leech might point out that earthworms have the great virtue of being more like leeches than humans are. Despite the Great Chain of Being's traditional ranking of humans between animals and angels, there is no evolutionary justification for the common assumption that evolution is somehow 'aimed' at humans, or that humans are 'evolution's last word'. It is remarkable how commonly this vainglorious assumption thrusts itself forward. At its crudest level, you meet it in the ubiquitously querulous, 'If chimps evolved into us, how come there are still chimps around?' I've already mentioned this, and I'm not joking. I meet this question again and again and again, sometimes from apparently well-educated people.*
5 'Monkeys [and other'higher' animals] are better at surviving than earthworms [and other 'lower' animals].' This doesn't even begin to be sensible, or even true. All living species have survived at least into the present. Some monkeys, such as the exquisite golden tamarin, are in danger of going extinct. They are much less good at surviving than earthworms are. Rats and cockroaches flourish, despite being regarded by many people as 'lower' than gorillas and orang-utans, which are perilously close to extinction.
I hope I've said enough to show what nonsense it is to rank modern species on a ladder, as though it were obvious what you meant by 'higher' and 'lower', and to show how thoroughly unevolutionary it is. You can imagine lots and lots of ladders; it might sometimes be sensible to rank animals on at least some of the ladders separately, but the ladders are not well correlated with each other, and none of them has any right to be called an 'evolutionary scale'. We have seen the historical temptation to crude errors such as 'Why aren't there any fronkeys?' But the pernicious legacy of the Great Chain of Being also feeds the challenge 'Where are the intermediates between major animal groups?' and, nearly as discreditably, underlies the tendency of evolutionists to answer such a challenge by trotting out particular fossils, such as Archaeopteryx, the celebrated 'intermediate between reptiles and birds'. Nevertheless, there is something else going on underneath the Archaeopteryx fallacy, and it is of general importance; so I shall give it a couple of paragraphs, using Archaeopteryx as a particular example of a general case.
Zoologists have traditionally divided the vertebrates into classes: major divisions with names like mammals, birds, reptiles and amphibians. Some zoologists, called 'cladists',* insist that a proper class must consist of animals all of whom share a common ancestor which belonged to that class and which has no descendants outside that group. The birds would be an example of a good class.^ All birds are descended from a single ancestor that would also have been called a bird and would have shared with modern birds the key diagnostic characters - feathers, wings, a beak, etc. The animals commonly called reptiles are not a good class in this sense. This is because, at least in conventional taxonomies, the category explicitly excludes birds (they constitute their own class) and yet some 'reptiles' as conventionally recognized (e.g. crocodiles and dinosaurs) are closer cousins to birds than they are to other 'reptiles' (e.g. lizards and turtles). Indeed, some dinosaurs are closer cousins to birds than they are to other dinosaurs. 'Reptiles', then, is an artificial class, because birds are artificially excluded. In a strict sense, if we were to make reptiles a truly natural class, we should have to include birds as reptiles. Cladistically inclined zoologists avoid the word 'reptiles' altogether, splitting them into Archosaurs (crocodiles, dinosaurs and birds), Lepidosaurs (snakes, lizards and the rare Sphenodon of New Zealand) and Testudines (turtles and tortoises). Non-cladistically inclined zoologists are happy to use a word like 'reptile' because they find it descriptively useful, even if it does artificially exclude the birds.
But what is it about the birds that tempts us to hive them off from the reptiles? What is it that seems to justify bestowing on birds the accolade of 'class', when they are, evolutionarily speaking, just one branch within reptiles? It is the fact that the immediately surrounding reptiles, birds' close neighbours on the tree of life, happen to be extinct, while the birds, alone of their kind, marched on. The closest relatives of birds are all to be found among the longextinct dinosaurs. If a wide variety of dinosaur lineages had survived, birds would not stand out: they would not have been elevated to the status of their own class of vertebrates, and we would not be asking any such question as 'Where are the missing links between reptiles and birds?' Archaeopteryx would still be a nice fossil to have in your museum, but it would not play its present starring role as the stock answer to (what we can now see is) an empty challenge: 'Produce your intermediates.' If the cards of extinction had fallen differently, there would just be lots of dinosaurs running about, including some feathered, flying, beaked dinosaurs called birds. And indeed, fossilized feathered dinosaurs are now increasingly being discovered, so it is becoming vividly clear that there really is no major 'Produce your missing link!' challenge to which Archaeopteryx is the reply.
Let's proceed, now, to some of the major transitions in evolution, where 'links' have been alleged to be 'missing'.
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