Our cousins the chimps
Each week I plan to present some new research or explore a different angle on evolution. Why evolution? Because it's the backbone of all the life sciences, it's a booming area of research, and it's increasingly being applied to medicine, ecology, and psychology.
Our cousins the chimps
Ted Siek from Southampton, Bucks County recently wrote me with an extensive list of questions about evolution. Some expressed a skeptical tone about science. I liked them because they challenge scientists to explain how evolution works.
For example: "If our DNA is 80 to 96 percent similar to apes and monkeys," Siek wrote, "why are humans so vastly different with respect to 1) life span, 2) morphology [form], 3) age to maturity, 4) intrinsic beauty, 5) walking, 6) strength, 7) speech, 8) healing rates, 9) mental capacity, 10) cognitive capacity?"
That seemed particularly relevant for this new weekly column called "Planet of the Apes." Each week I plan to present some new research or explore a different angle on evolution.
Why evolution? Because it's the backbone of all the life sciences (a big enterprise here in Philadelphia), it's a booming area of research, and it's increasingly being applied to medicine, ecology, and psychology. And because many readers, including Siek, say they want to learn more about it.
Our genetic similarity to apes that Siek mentions turns out to highlight one of the most powerful pieces of supporting evidence for evolution.
Darwin postulated that all living things are blood relatives, linked in one big family tree. We humans share a branch with other apes; that branch is attached to a bigger limb with other primates, and a still bigger limb with other mammals.
That's exactly what DNA comparisons have shown. We have many genes in common with all other living things..
Those genetic comparisons reflect our relationships on the family tree. If we'd found that we were 98 percent genetically identical to fruit flies or mushrooms and shared only 60 percent of our DNA with chimps, then Darwin's adherents would have some explaining to do.
Comparing our genes to other species has led to new insights: Our genetic differences with chimps may seem small, but we've learned that some genes act on one another, creating a cascade of effects that ripple through life, leaving us with bigger brains and less hair. So even a small genetic divergence can lead to big differences because of the way genes work.
It turns out we humans are different from chimps in ways that go beyond Siek's list. Last month, for example, a group of biologists announced a DNA difference apparently responsible for the fact that human males lack "penis spines" - whiskery appendages that adorn chimp penises and those of many other mammals.
The DNA shows that the chimpanzee is our closest relative, and our genetic codes are 96 percent to 98.5 percent alike, depending on how the analysis is done. That means we're genetically closer to chimps than chimps are to gorillas.
Our genetic similarities to chimps came as a surprise to Mary-Claire King, a biologist who made one of the first comparisons back in 1975. At the time she was a graduate student at the University of California at Berkeley, working for the late professor Alan Wilson.
Lacking the tools scientists now use to read DNA, they relied on more indirect methods of comparison, including sticking chimp and human DNA together and measuring how much heat would be added to cause the strands to fall apart. The more similar the DNA strands, the harder they tended to stick to each other.
Wilson and King's first results showed no difference whatsoever - comparing chimp DNA to human was like comparing two different humans. More statistical analysis led them to conclude our genes were nearly identical.
King and Wilson speculated at the time that the reason we walk upright and talk has to do with the way our genes turn on and off.
That turned out to be prescient, said Scott Gilbert, a developmental biologist at Swarthmore College. Different-looking animals can carry very similar genes, he says, if there are big differences in the way these genes are switched on and off in utero and in infancy.
When chimps are born, their brains are pretty much done developing, he said, while human infants are helpless and have brains that continue to develop rapidly for several years and then more gradually into the teens.
It took several decades for scientists to start isolating the specific genes where our codes differ from those of chimps.
The first was discovered by Ajit Varki, a physician-scientist at the University of California at San Diego. The gene in question caused humans to lack a kind of sugar called Neu5Gc, which was thought common to all mammals.
Varki thinks that missing sugar may explain why epidemiologists keep finding more cancers and other disease among those who eat lots of meat from our fellow mammals. Red meat is full of Neu5Gc, he said, which triggers inflammation, since our bodies react to it as a foreign invader.
It's enough to persuade him to stick to fish and chicken.
In 2009, scientists made another discovery, announcing that humans and chimps carried different versions of a gene called FOXP2. In humans, mutations in that gene can cause a speech deficit and some problems with reading and writing, suggesting a link between FOXP2 and language.
UCLA brain scientist and geneticist Daniel Geschwind followed up on this, and found that FOXP2 is more than just language-related. It seems to act as a master switch controlling hundreds of other genes, many of them important in the brain.
So a difference in one gene can have a cascading effect on others. It was just as King and Wilson had speculated in 1975.
Just last month, another group of scientists announced they'd found more than 500 additional differences between chimp and human DNA. These were not just simple spelling errors but whole stretches of DNA that are present in mice, chimps, and monkeys but missing in humans.
Interestingly, they were nearly all found in regions of DNA that had been labeled "junk" because it doesn't contain any genes.
Now we know that much of this so called "junk" DNA has important jobs, said Philip Reno, a Pennsylvania State University anthropologist and part of the team announcing these missing pieces.
They analyzed two in detail, he said, and found both of them important in orchestrating development in utero. One spurs the growth of both sensitive whiskers and those penile spines, he said. Most of our fellow mammals have facial whiskers with tactile sensation - larger ones in cats and mice, smaller ones in chimps.
And the penis spines are a feature of male mice, cats, monkeys, chimps, and many other mammals.
They're made out of the same material as fingernails, Reno said, but in chimps, they're only about 1mm long. Is that good or bad for the female? It's hard to know, he said, but chimp intercourse takes only 10 seconds, so maybe it doesn't make that much difference.
He speculates that since the spines are sensitive to touch, they're stimulating for males and may help them get their sperm out quickly.
We humans still carry the penis spine and whisker gene, but - lucky us - somewhere along the line we lost the on-switch.
Why did human males shed their spines? It's possible humans benefited from longer intercourse because it encouraged bonding in relationships, Reno said, or it's possible that females preferred males without them so only the spine-less got to procreate.
That leaves out a few parts of Siek's inquiry, including the issue of longevity. Other new research shows our fellow primates don't age as fast as we thought, but that will have to wait for another column.
I won't sign off, however, before taking on that question Siek posed about differences in "intrinsic beauty."
For that, I'll turn not to the real experts but to the fictional one portrayed in the original 1968 Planet of the Apes. Near the end, the youthful, strapping Charlton Heston character asks for a goodbye kiss from his chimp benefactor, the behaviorist Dr. Zira. The female chimp eyes the young Heston with revulsion. "All right, but you're so damned ugly."