The ancestors of modern humans evolved to grow quickly in the womb, according to a new study. It’s hard to find evidence of pregnancy in fossils, so researchers turned to a mathematical model. They inferred the growth rate in the womb across primates from the skull volume of 19 primate groups (including adult and juvenile Australopithecus, chimps, and Homo erectus, shown in the image above), as well as from the proportions of the first and third molars of 608 primates, New Scientist reports. Hominins—which includes our species and other extinct human species—started to increase their growth pace in the womb over the past 6 million years—around the time they split from chimps, the researchers report this week in the Proceedings of the National Academy of Sciences. Ancient humans achieved a growth rate in the womb similar to modern humans less than 1 million years ago, even before modern humans emerged. The scientists say this rapid growth during pregnancy might have been crucial for evolving the big brains we have today.