These days, the idea of mingling closely with our evolutionary cousins is best left to movies like War For the Planet of the Apes, but humans didn’t always feel that way. According to a recent genetic analysis of modern humans, our ancestors who lived in Africa thousands of years ago got it on with early hominin species that weren’t Homo sapiens. Evidence of these carnal trysts, found in samples of their saliva, presents the unpalatable possibilty that interbreeding wasn’t just a random occurrence — but the norm.

But exactly which ancient hominin the Homo sapiens in the study decided to bone, however, remains a mystery — and it’s very likely that these “ghost” humans haven’t yet been discovered in the fossil record.

“This unknown human relative could be a species that has been discovered, such as a subspecies of Homo erectus, or an undiscovered hominin,” study lead and assistant professor of biological sciences at the University of Buffalo, Omer Gokcumen, Ph.D., said in a statement. “We call it a ‘ghost’ species because we don’t have the fossils.”

In a new paper published in Molecular Biology and Evolution, Gokcumen and his team explain that the ancient saliva samples they examined contained a protein called MUC7, which opened up the possibility of the hominin “ghosts.” It’s likely that ancestors of people who carry a specific variant of the MUC7 gene, they argue, interbred with the ghost species, who peeled off from Homo sapiens’ evolutionary path as far back as two million years ago or as recent as 150,000 years ago.

Analyzing 2,500 modern human genomes revealed that some ancient humans from Sub-Saharan Africa had a version of MUC7 that was way different from that of other humans. These variants were so unusual, in fact, that when the scientists compared them to equivalent MUC7 genes in other ancient hominin species — Neanderthals and Denisovans — they found that those genes more closely matched the genetics of modern humans than the Sub-Saharan version.

New species, who this?

This insight into the inter-breeding habits of early humans beyond Europe and Africa became even more surprising when the scientists investigated the function and origin of the MUC7 protein. The most abundant protein in human saliva, MUC7 helps us swallow, chew, speak, and keep the mouth clean of bacteria. But different versions of the genes also determines the composition of a person’s oral microbiome.

Why this gene variant matters — other than showing that ancient humans liked to swap spit with literally unknown strangers — is that it adds support to a less accepted hypothesis about the origin of the human species. Scientists accept that Earth was once, as evolutionary geneticist Mark Thomas told Nature in 2013, a “Lord of the Rings-type world,” with “many hominid populations.” What is less understood is exactly how modern humans came to be: The most accepted explanation is the “Out of Africa” theory, which posits that Homo sapiens left Africa between 125,000 and 60,000 years ago, and over time replaced the “primitive” populations of Asia and Europe.

But a competing theory, which the newly uncovered sexually adventurous hominins support, is that all archaic human forms, like the “ghost” species and Neanderthals, evolved together into populations of Homo sapiens, eventually transforming into the categorically different form of Homo that makes up our world today. There’s no confirmed, exact version of how moderns humans came to be yet — but discoveries like this variant of MUC7 bring us one step closer to untangling the branches of our twisted family tree.