In 2011, archaeologists discovered cremated bones on a hearth at the site. Research revealed that the bones belonged to a 3-year-old child. Below the hearth, the team discovered a burial pit containing the skeletons of two other children.

One of the buried children was an infant who died a few months after birth; the other was likely a late-term fetus. After the baby and the fetus died, their bodies were carefully laid atop a bed of red ocher, surrounded by antlers fashioned into hunting darts.

“These things we hardly ever find — it’s a very rare window into the worldview of these people,” said Ben A. Potter, an archaeologist at the University of Alaska Fairbanks who has led the research at Upward Sun River.

Clearly, the children had been ceremonially buried. But why the two bodies ended up in the same grave is impossible to know. With the consent of Native American tribes that live around the site, Dr. Potter and his colleagues drilled small pieces of bone from the two skeletons and sent them to geneticists at the University of Utah.

The vast majority of our DNA lies in the nucleus of the cell. But the cell’s energy factories, called mitochondria, also carry small bits of their own DNA inherited solely from our mothers.