The extinction of Neanderthals remains one of paleoanthropology’s most debated mysteries. Climate change, competition with Homo sapiens, low birth rates, disease, and even cannibalism have all been suggested as possible causes. Now researchers have proposed another, rather unexpected factor: life-threatening pregnancy complications — primarily preeclampsia and eclampsia. The study presenting this hypothesis was published in the Journal of Reproductive Immunology.

Why Preeclampsia Is Considered a “Human” Disease

Preeclampsia is a severe pregnancy complication linked to abnormal placental development. The mother experiences a sharp rise in blood pressure, protein appears in the urine, and the fetus is put at risk due to insufficient blood supply. In the most serious cases, the condition progresses to eclampsia, which can cause seizures, coma, and a high risk of death for both mother and child.

Remarkably, among more than 4,300 mammalian species, clinically significant preeclampsia is found only in humans. In other primates and mammals, the placenta typically implants more deeply and securely, largely preventing such complications.

The authors of the new study suggest that Neanderthals likely had placental implantation mechanisms very similar to those of modern humans — meaning they may also have been vulnerable to preeclampsia. However, Homo sapiens appear to have developed protective evolutionary mechanisms over hundreds of thousands of years that usually prevent mild disorders from progressing into severe cases. Neanderthals, it is believed, lacked these safeguards.

Estimates: How Many Women Might Have Died?

Using population modeling, the researchers concluded that without modern protective mechanisms, the rate of severe preeclampsia and eclampsia could have reached 10–20% of all pregnancies. Mortality among first-time mothers — who are particularly vulnerable — may have been around 4%.

For small, isolated Neanderthal groups (estimated to range from a few hundred to a few thousand individuals spread across vast territories), such losses would have been catastrophic. Even a modest decline in fertility and maternal survival could have led to irreversible population shrinkage within just a few generations.

Genetics May Also Have Worked Against Neanderthals

Genetic evidence indirectly supports the hypothesis. Modern humans who inherited certain Neanderthal gene variants — particularly near the H19 gene — show a higher risk of hypertension during pregnancy. Other Neanderthal alleles related to immune interactions between mother and fetus may have contributed to shallower placental implantation, a major risk factor for preeclampsia.

The situation may have worsened after interbreeding with Homo sapiens. Hybrid pregnancies (a Neanderthal mother carrying a child with sapiens genes) could have increased immune incompatibility, further raising the likelihood of complications.

Not the Only Cause — but a Significant One

The researchers emphasize that preeclampsia and eclampsia were unlikely to be the sole cause of Neanderthal disappearance. Rather, they were probably one factor among many, acting alongside climate stress, competition for resources, low population density, and possibly cultural differences.

However, the emergence of evolutionary protections against preeclampsia may have given Homo sapiens a decisive reproductive advantage. The same adaptations that helped our ancestors survive despite historically high maternal risks may also have reduced pregnancy-related losses.

In Brief

A new hypothesis published in the Journal of Reproductive Immunology suggests that pregnancy complications — specifically preeclampsia and eclampsia — may have played a significant role in Neanderthal extinction. Lacking the protective mechanisms found in Homo sapiens, severe cases may have affected 10–20% of pregnancies, with maternal mortality reaching about 4%. Genetic factors and immune incompatibility during interbreeding could have worsened the problem. While not the sole cause of their disappearance, these complications may have critically reduced birth rates and survival in already small Neanderthal populations.