The issue of interpretation, what does this result mean?, can still be controversial, even if the result itself can be made precise. For example, S. Blair Hedges and Sudhir Kumar have applied the DNA clock to 658 genes in 207 species of modern vertebrates: rhinos, elephants, rabbits, and so on. Their results suggest that many of these lineages were around at least 100 million years ago, coexisting with the dinosaurs, though no doubt the early elephant and rhino ancestors were rather small. The fossil record agrees that there were mammals then, but not those. The molecular biologists claim that the fossil record must be misleading; palaeontologists are convinced that the DNA clock sometimes ticks faster and sometimes ticks slower. The debate continues, but for what it's worth, our money is on the palaeontologists.
One big surprise about mammal DNA is how much of it there is. You might expect a sophisticated creature like a mammal to be 'hard to build' and therefore require more DNA, just as the blueprint for a jumbo jet has to be more complicated than that for a kite.
Not so.
Mammals have less DNA, shorter genomes, than many apparently simpler animals, for example frogs and newts.
There's a good reason for this apparent paradox, and it illuminates the difference between DNA and a blueprint. DNA is more like a recipe, and a recipe that makes a lot of assumptions about what else you have in your kitchen, so that none of that needs to be spelled out in the recipe book. In essence, the kitchen for mammals has a really well controlled oven, capable of ensuring nice, even cooking temperature, so a whole lot of tricks about what to do if the temperature changes need not be mentioned. In the frog kitchen, on the other hand, the temperature goes up and down depending on the time of day and the weather, so the recipe has to deal with all contingencies, requiring more DNA code. By 'kitchen' here we mean the environment in which the embryonic animal has to develop. For a frog, the kitchen is a pond. For a mammal, the kitchen is mother.
Mammals evolved good temperature control, unlike the reptiles, they are warm-blooded, but what matters is not so much being warm, as being controllable. Frog DNA is full of genes for making lots of different enzymes, together with instructions along the lines of 'use enzyme A if the temperature is lower than 6°C, use B if the temperature is between 7°C and 11°C, use C if the temperature is between 12°C and 15°C ...' Mammal DNA just says 'Use enzyme X', knowing that mother will take care of temperature variations. Frog DNA is a rocket: mammal DNA is a space elevator.
How did this change take place? Perhaps when mammals first evolved, their DNA gained extra instructions, but after temperature control evolved, a lot of the DNA became redundant, and it either got dumped or got subverted to other uses. On the other hand, we have no idea what the DNA of early mammals actually looked like, maybe it was all shorter in those days, maybe today's frogs and newts have much more extensive recipes than ancient ones. But on balance it seems more likely that mammals just eliminated a lot of surplus instructions.
Modern technology uses the same trick. Because the machinery that makes today's consumer goods is extremely precise and accurate, those goods can be simpler than they were in the past. A soft drinks can, for example, is little more than a piece of aluminium that has been formed into a cylinder, with another flat bit on top to act as a lid, a weak line for the tab to tear along, and a ring (or nowadays a lever) attached to the tab. It replaces the bottle, which consisted of two or more bits of moulded glass 'welded' together, a metal cap, and a slice of cork. The simplicity of the can comes at a price: very careful control of the forming process.
There are many scientists who insist that an organism's DNA determines everything about it, even though it manifestly does not, and they argue that the mother's temperature-control system is included in her DNA recipe. This may well be true, but even if it is, 'this organism's' DNA has somehow migrated to another organism (mother, not her offspring). As soon as two generations are involved in implementing the genetic blueprint, a gap opens up into which things can be inserted that are not genetic at all. We've already mentioned several, for example prions in the reproduction of yeast.
Our mammalian ancestry may even be responsible for one of the more bizarre modern myths, persistent tales of people being abducted by aliens. Ufologists allege that one American in twenty now claims to have undergone such an experience (but they would, wouldn't they?). If true, this figure is a remarkable and not very happy comment either on the critical faculties of that great nation or on the habits of an unknown spacefaring species. Be that as it may, a lot of people are convinced that strange aliens, usually with big black eyes and pear-shaped heads like the ones in Close Encounters of the Third Kind, landed a UFO near them, loaded them on board, and took them for a flight round the solar system while carrying out weird experiments, often of a sexual nature, on them. After which they were calmly returned to the very spot from which they had been abducted, as if absolutely nothing had happened.
The first thing to say is that without doubt many of these experiences are false. Ianonce did a radio broadcast which included a woman who had undergone a convincing experience of being abducted, except that she knew she hadn't really been, because her family told her she'd been asleep beside the fire the whole time. Jack once met a woman who claimed that the aliens abducted her and took away her baby. So he asked a question that nobody else had thought to ask, the woman included: 'Were you pregnant?'
'No.'
The point is that to the victims, the experience felt real. Even though logic told them it couldn't have happened, they either didn't apply the logic, or they did but still remembered the experience vividly. We deduce that the human mind sometimes has vivid memories that do not correspond to real events. Of course we must also observe that just because some alien abductions aren't real, that doesn't imply they all aren't. However, if we can find a sensible mechanism for otherwise reasonable people believing that they really were carted off in a UFO, then the burden of proof shifts dramatically and evidence of abduction stronger than sincere expressions of belief becomes necessary.
Reports of alien abductions are not new. Back in the Middle Ages, however, they would have been either flights on witches' broomsticks or encounters with fabulous creatures like the succubus, a demon in a woman's body who allegedly had sex with men while they slept. The witches of Discworld employ broomsticks for transport only. The sex bit doesn't appeal to them at all, except for Nanny Ogg, of course.
Folk tales of succubi and their like can be found worldwide. In Newfoundland people tell of an ancient hag sitting on their chests at night, and in Vietnam they speak of the 'grey ghost'. What seems to be going on is some common mental pattern, overlaid with cultural influences. That's why abductions by witches riding broomsticks have gone out of vogue, but abductions by aliens riding UFOs are flavour-of-the-decade.
Susan Blackmore thinks that all of these experiences are, and were, caused by 'sleep paralysis'. This is a feature of the mind that prevents sleeping people from moving their limbs as they would if they were acting out their dreams. Such a 'mental switch' is important for any animal that dreams: you don't really want to go sleepwalking out of your cosy burrow and straight down a predator's throat. Plenty of mammals dream, most of us have seen a cat or dog asleep with its legs twitching, and the evidence from recordings of the brain's electrical activity is that the animals are engaged in something that closely resembles the brain activity of a dreaming human. We can't be sure whether cats have visual dreams like we do, but sleep and dreaming take place in primitive parts of the brain, so they probably go back a long way in our evolutionary history. At any rate, if the sleep paralysis system malfunctions, people who are partially awake may undergo sleep paralysis. Experiments show that in such cases they typically get a strong impression that 'somebody is there'.