Our ancestors branched off from those of chimpanzees some six million years ago. Since then, our lineage became human–and distinctly unlike other apes. Figuring out how that difference evolved is one of the grand challenges of biology. Until now, scientists have gotten most of their clues by looking at the fossils of extinct hominids. These fragments of bones only preserve a little information, but it’s not a random smattering of data. It’s more like a scaffolding on which other clues can be fixed, so that a picture of how we became human can gradually emerge. That’s because the changes documented in the fossil record were ultimately created through the evolution of our genome.
The power of combining fossils and genes was demonstrated today in Nature. Scientists at the University of Pennsylvania reported their studies on a human gene that has mutated into uselessness. Such broken genes are nothing new; scientists have identified several hundred broken genes in the human genome dedicated to smell alone. What’s striking about this particular gene, known as MYH16, is how important it is in its functional form to our primate cousins. MYH16 is a muscle-building gene that only becomes active in the developing muscles of the jaw. Apes and monkeys all have massive jaw muscles, which pass from the jaw up under their cheekbones, fanning out across the top of their skull and anchoring to a keel-shaped ridge. We have no such ridge, and we have pretty puny chewing muscles compared to apes. And much of the difference between us and other primates in this respect comes down a fatal mutation that hit a single gene: MYH16.
To get a better picture of this crucial event in our evolution, the Penn team compared our version of the gene to those found in primates and other mammals. By tallying up the changes each version of the gene underwent, they were able to estimate when MYH16 shut down in our own lineage. Their estimate: 2.4 million years ago.
This was no ordinary time in our history. Hominids before that age still had big anchoring crests and large jaws. Younger species had smaller jaws and smooth tops on their skulls. And something else happened at the same time: their brains began getting significantly bigger. It’s possible that when MYH16 still worked in our ancestors, their chewing muscles acted like a clamp on the evolution of brain size. The architecture of the entire top of the head was so dominated by the muscles and their anchoring that expanding the brain was impossible.
As fascinating as this result is, it’s important to keep in mind that we’re just at the beginning of this fusion of genes and fossils. Today’s report is actually the first to link some important fossilized trait with the evolution of a human gene. A few other studies have revealed some other genes that were also important in human evolution, although they leave a subtler mark on the hominid line. The language gene FOXP2 appears to have undergone intense evolution perhaps 100,000 years ago. There are probably several thousand genes that have evolved significantly since we parted ways with our fellow apes, and so it’s easy to blow these early discoveries out of proportion. While MYH16 may well have played a crucial role in our evolution, it didn’t do it alone.
For one thing, a hominid with a weak jaw can’t grind up tough foods the ways its ancestors did; it needs new foods. The oldest tools, interestingly enough, date back to about the same time as the MYH16 mutation. Scientists suspect that hominids were using these simple stone axes to hack meat off of carcasses and dig up tubers. This new diet might have meant that a mutation to MYH16 wouldn’t have mattered much. The new diet may have been just as important as the missing jaw muscles to letting the hominid brain expand. For one thing, a big brain requires lots of energy. One way to make more energy available is to shrink the size of other organs, and it turns out that we humans have one particulary small organ: our intestines. Other primates use their long bowels to digest tough foods poor in nutrients; we can survive on our abbreviated bowels because we eat better grub. So here’s a prediction: scientists will eventually discover genes that control the development of intestines in humans. When they compare them to ape genes, they’ll discover that they underwent an evolutionary change around the same time that MYH16 shut down. Our brains did not evolve in a vacuum; they coevolved with the rest of our bodies in a complicated dance of tradeoffs and feedbacks.
(I don’t want to turn every post about evolution in an attack on creationism, but here’s a parting question. MYH16 is clearly essential to the well-being of other primates. We have a copy of MYH16, but it doesn’t work. Where is the intelligence of this design? If we don’t need the gene, why did the designer insert it into our genome?)
Originally published March 24, 2004. Copyright 2004 Carl Zimmer.