One of the most interesting things about Homo sapiens is that it is the only living member of the genus Homo. In other words, our species doesn’t have any brothers or sisters.

The fossil record, however, indicates that in the past, many more species of humans existed.

Recently discovered fossils of Homo rudolfensis—two jawbones and the lower part of a face—indicate that this was almost certainly a separate species that lived in Africa alongside other species like Homo habilis and Homo erectus.

Homo floresiensis lived on the island of Flores in Indonesia as late as 13,000 years ago; Homo sapiens lived at the same time.

Neanderthals lived alongside modern humans. They may have interbred with us. Some scientists think that Neanderthals were a subspecies of Homo sapiens and not a separate species.

What it would be like to live in a world where Homo sapiens is not the only species of human?

Would it change our views about equality and justice?

Today, we sometimes make decisions about how to treat other people based on their race, but race is a concept created by societies—there are no true, genetically distinct living human subspecies or races.

Suppose, however, we were to share our world with people who can use tools, have some form of language and can think abstractly, but aren’t exactly like us. Perhaps their brains are slightly less (or more) developed. Maybe their language skills are less advanced, but they have superior spatial intelligence.

There are definite physical and psychological differences, however small, that cannot be explained away as cultural differences and that can be identified in our DNA.