The researchers who discovered the skull of a tiny human in Indonesia two years ago have now published a report on the ancient remains of eight similarly sized hominids. The scientists have argued that the specimens represent a new species, but skeptics think the skulls and bones come from normal humans whose small size might be explained by either microcephaly or a form of island dwarfism. What are microcephaly and island dwarfism?
One is a rare condition in which a person develops with a tiny brain; the other describes a population of animals on an island that shrinks in size over thousands of years. Doctors call an infant "microcephalic" if the circumference of its head is two or three standard deviations below the average for comparable babies. In "primary microcephaly," the brain does not develop to its normal size while the baby is still in the womb. (Researchers say there are about six cases of primary microcephaly for every 10,000 births in the United States.) In "secondary microcephaly," the brain appears normal at birth but fails to grow and form connections at a normal rate.
Some forms of microcephaly are genetic. Other types may be caused by infections or viruses like herpes, syphilis, and HIV. Expectant mothers who drink or smoke also increase their risk of having a microcephalic baby. The disorder can produce brains of variable sizes and textures. As a result, people with microcephaly can have varying degrees of mental retardation, along with seizures and other problems. Some have abnormally small bodies as well. (Last year, tiny-human doubters argued that the skull from Indonesia came from a microcephalic with reduced body size.)
The evolutionary phenomenon of island dwarfism—and the parallel concept of island gigantism—was first identified in the 1960s. Biologists noticed that animal remains found on islands tended to be either significantly larger or significantly smaller than the remains of similar animals from the mainland. As a general rule, populations of bigger mammals seemed to shrink in size when they got stuck on an island, while smaller ones grew. On the island where the tiny human remains were found, researchers have also found evidence of giant rats and little elephants.
What causes island dwarfism? It's not entirely clear, but limited resources likely play a role. Isolated animals have less access to food, and malnourished animals are less likely to have offspring. Since smaller creatures can get by with smaller meals, they would be naturally selected to reproduce.
Bonus Explainer: Much has been made of the fact that neither of the two tiny jawbones discovered on the Indonesian island showed evidence of chins. Why is this important? Skeptics argue that the bones come from modern humans who happened to be very small, and that at least one of the specimens shows signs of microcephaly. But since only modern humans (Homo sapiens) have chins, the bones may have come from humans that descended from an earlier species. No one knows why we evolved chins, but one theory suggests that it helped to reorient our teeth after our faces evolved to become shorter and flatter.
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Explainer thanks Robert Eckhardt of Pennsylvania State University, Dean Falk of Florida State University, and Daniel Lieberman of Harvard University.