Junk food or sedentary lifestyles have been blamed for the current obesity epidemic, but long before fast-food chains or small-screen addiction came on the scene, evolution was conspiring to make us the fattest primates on the planet.
Though we share 99 percent of the same genes with chimpanzees and many other primates, we are far more fat than any of them. Now, researchers at Duke University in the United States say they have found evidence of why we as a species ended up becoming chubbier than chimpanzee.
According to Duke scientists, despite having nearly identical DNA sequences, chimps and early humans underwent critical shifts in how DNA is packaged inside their fat cells. This difference in DNA packaging within fat cells decreased the ability of humans to convert ‘bad’ calorie-storing fat into the ‘good’ calorie-burning kind.
Compared to our closest animal relatives, even people with six-pack abs and rippling arms have considerable fat reserves. While other primates have less than 9 percent body fat, a healthy range for humans is anywhere from 14 to 31percent.
In their study, researchers compared fat samples from humans, chimps and a more distantly-related monkey species, rhesus macaques. Using a technique called ATAC-seq, they scanned each species’ genome for differences in how their fat cell DNA is packaged.
Normally most of the DNA within a cell is condensed into coils and loops and tightly wound around proteins, such that only certain DNA regions are loosely packed enough to be accessible to the cellular machinery that turns genes on and off.
The researchers identified roughly 780 DNA regions that were accessible in chimps and macaques, but had become more bunched up in humans. Examining these regions in detail, the team also noticed a recurring snippet of DNA that helps convert fat from one cell type to another.
Saying that not all fat is created equal, the researchers explain that most fat is made up of calorie-storing white fat. This white fat, distinctly visible as marbling in steak, builds up around our waistlines. Specialized fat cells called beige and brown fat, on the other hand, can burn calories rather than store them to generate heat and keep us warm.
One of the reasons we are so fat, the research suggests, is because the regions of the genome that help turn white fat to brown were essentially ‘silenced’ in humans but not in chimps.
Nevertheless, the researchers say it is still possible to activate the body’s limited brown fat by doing things like exposing people to cold temperatures, Humans, like chimps, need fat to cushion vital organs, insulate us from the cold, and buffer us from starvation. But early humans may have needed to plump up for another reason, the researchers say — as an additional source of energy to fuel our growing, hungry brains.
In the six to eight million years since humans and chimps went their separate ways, human brains have roughly tripled in size. Meanwhile, chimpanzee brains have not budged in size.
The human brain uses more energy, pound for pound, than any other tissue. Steering fat cells toward calorie-storing white fat rather than calorie-burning brown fat, the thinking goes, would have given our ancestors a survival advantage.
Because of brown fat’s calorie-burning abilities, numerous researchers are trying to figure out if boosting our body’s ability to convert white fat to beige or brown fat could make it easier to slim down. Maybe we could figure out a group of genes that we need to turn on or off, but we are still very far from that, admitted the researchers.