Momentarily in non-stick figure form, hands and all, to bring you four fascinating stories about how we humans have evolved. Speaking of which, there’s something fishy about how humans evolved these hands in the first place. You’ve probably heard that every bony land-dwelling creature on Earth evolved from fish that grew limbs and crawled out of the water many million years ago. And yet, over 99% of the fish in today’s oceans, lakes, and rivers have incredibly delicate fins, formed by a series of spines covered by a thin web of skin.
It’s hard to imagine fins like those evolving into sturdy, weight-bearing limbs…because they didn’t. But the fins of a far more foreign fish did. Very soon after bony fishes first evolved, they branched into two groups – those with flimsy ray-type fins, and those with more meaty appendages that were attached to their bodies by a single strong bone. At first, those so-called lobe-finned fishes far outnumbered their ray-finned relatives, but just as some of them were becoming adapted for life in the weedy shallows, global climate fluctuations caused a mass extinction that devastated life in the oceans.
After the catastrophe, the ray-finned fishes recovered and took over the oceans. The lobe-fins never recovered, but a few survivors in the shallows managed to find their footing out of the water, and eventually gave rise to every backboned, four-limbed creature to ever walk, hop, crawl, or fly on land. One of those four-limbed vertebrates, of course, is the chimpanzee, one of our closest living relatives – but how close are we, exactly? It’s often said that we humans share 50percent of our DNA with bananas, 80 per cent with dogs, and 99 per cent with chimpanzees.
Taken literally, those numbers make it sound like we could pluck one cell from a chimp and one from a human, pull out the tangled bundles of DNA known as chromosomes, unroll each one like a scroll, and read off two nearly identical strings of letters. But in reality, the human and chimp scrolls don’t sync up so easily. In the six to eight million years since we split from our last common ancestor, chance mutations and natural selection have changed each of our genomes in radical – and unique – ways. Two human scrolls fused, leaving us with 23pairs of chromosomes to chimps’ 24. Other large mutations revised huge sections of text – duplicating a chunk of a human DNA here, erasing a chunk of chimp DNA there– while, throughout the scrolls, tiny mutations swapped one letter for another.
When researchers sat down to compare the chimp and human genomes, those single-letter differences were easy to tally. But the big mismatched sections…weren’t. For example, if a genetic paragraph – thousands of letters long – appears twice in a human scroll, but only once in its chimp counterpart, should the second copy count as thousands of changes, or just one? And what about identical paragraphs that appear in both genomes, but in different places, or in reverse order, or broken up into pieces? Rather than monkey around with these difficult questions, the researchers simply excluded all the large mismatched sections – whopping1.3 billion letters of DNA – and performed a letter-by-letter comparison on the remaining2.4 billion, which turned out to be 98.77% identical.
So, yes, we share 99% of our DNA with chimps- if we ignore 18 per cent of their genome and 25 per cent of ours. And there’s another problem: just as a small tweak to a sentence can alter its meaning entirely or not at all, a few mutations in DNA sometimes produce big changes in a creature’s looks or behaviour, whereas other times lots of mutations make very little difference. So just counting up the number of genetic changes doesn’t really tell us that much about how similar or different two creatures are. But that doesn’t mean we can’t learn anything by comparing their genomes. DNA contains a record of the evolutionary relationships between all organisms. It’s a garbled record – but by reading closely, we’ve been able to glean enough information to refine the evolutionary trees we started drawing long before genome sequencing was around.
We may not actually be 99 per cent chimp, but we are 100 per cent great ape…and at least a little bananas. We may be a little bit bananas, but some of what we do does make sense – at least from an evolutionary point of view. Like…that reaction you just had. It turns out that, long ago, snakes may have coiled their way right into our brain. Our early ancestors were tasty snacks for everything from carnivorous mammals to birds of prey; luckily, they had developed a basic predator pattern recognition system that quickly located general threats and gave the primate time to escape. But when predatory snakes first came on the scene, that system totally failed. That’s because snakes are essentially stick-look-alike whose hundreds of rib muscles and huge ventral scales help them move in a way that no other predator does, which kept them incognito and helped them chow down on unsuspecting prey. But eventually, some of these primates evolved what amounted to snake detectors – adaptations like neurons devoted specifically to snake-spotting- that helped the protomonkeys quickly recognize a wriggling stick as dangerous and get out of Dodge. Eventually, some of those helpful snake-sensing adaptations got passed down all the way to humans. Tests that measure the electrical activity in our brains show that when we look at pictures of snakes, we experience lots of activity right at the point when the brain is evaluating threats – far more than when we’re looking at pictures of other predators.
And take a look at these sequences – you’ll probably see the creature on the top in an earlier frame than you see the one below, because your brain needs less visual information to pick out a snake than to identify other animals. Some researchers even think that we evolved our awesome color vision, in part, to help us successfully spot snakes. And there’s some evidence that our sensitivity to snakes actually makes us prone to fearing them, but we don’t yet know exactly how. What is clear, though, is that we definitely have snakes on the brain. I really like snakes. I don’t know, maybe that says something about my odds of survival. But it turns out that there’s something odd about all our odds of survival, once we reach a certain age. Not to sound super morbid, but with each passing second of life, the cells in our bodies accrue a little more damage, which is at least partly why, as people get older, their odds of dying increase: today, a 40 year-old has a .3% chance of dying in the next year, while a 60-year old has a 1% chance of dying in the next year, an 80 year old has a 5 per cent chance, and a 100-year-old has a 50% chance. But around that age, there’s some evidence that the odds of dying level off. So, what would make the mortality curve flatten out like that? Well, ne theory is that it has to do with natural selection and bad jeans – the other kind.
Take a hypothetical gene mutation that proves fatal during childhood. Because it kills its host before they get old enough to reproduce, it never gets passed on – natural selection weeds it out. On the other hand, a nasty mutation that tends to kill people after they grow up and reproduce can get passed on, but here’s where things get complicated, because human kids depend on their parents for survival. So a bad gene that tends to kill relatively young adults is also likely to indirectly kill their young kids, and thus be weeded from the gene pool, too. But a bad gene that kills slightly older adults has a lower chance of indirectly killing their slightly older, more self-sufficient kids, and thus has a lower chance of getting weeded out of the gene pool, and a gene that kill seven older adults has an even lower chance of also killing their kids.
In short, natural selection’s ability to eliminate harmful genes gets lower and lower as the age at which those genes strike gets higher and higher. And this could explain the weird death rate curve. Like, think of natural selection as a magical force field that protects people from the grim reaper, but it gets weaker as people get older, so their odds of dying go up. Eventually, the force field wears off entirely, so while people’s chances of dying at that age are really high, they don’t go up any further because there’s no protection left to lose. And if the death rate really does level out- say, somewhere around 50 per cent once we hit 100 years of age – how long humans can ultimately live is just a numbers game: right now, there are about half a million hundred-year-old son Earth. If half of them die each year, only about15 thousand will make it to 105, only about 500 will make it to 110, and odds are none of the current centenarians will beat the old-age record of 122 . But if we count all7.7 billion people living on the planet today, odds are that 15 people will beat that record– and one lucky human will live to the ripe old age of 127. I guess I’d be ok with 127, so long as it’snot 127 years of social distancing. I’m a total sucker for evolution, but this pandemic has left me stuck at home taking care of two kids, two cats, a dog, and a horse- and that’s on top of actually working full-time. So while my evolution-themed reading list is a mile long,