We humans like to think we're special, so different from all the other species. We tend to see it in black-and-white – either you are a person, or you aren't. But in human evolution, it wasn't black-and-white. There was no one point in time when we became human. Instead, different traits of humanity appeared at different times, and depending on what you think is most crucial, you'd draw the line at different points. Compassion for others and basic tool use were most likely present before our ancestors split off from chimps and bonobos. Upright walking and smaller jaws distinguished Lucy and other australopithecines from the other apes, but their brain was mostly unchanged. Then brain sizes increased, and the first stone tools were found, skillfully crafted by homo habilis. At first, our ancestors only made one kind of tool, but then we had an explosion of tool-making diversity, and we started to make technological advancement, with a steady improvement in tool designs over time. Then, deliberate burials, carved statues and cave paintings began to appear, suggesting the birth of imagination and religion.
There is a lot of disagreement over when certain crucial human behaviours appeared. It used to be thought that homo habilis was the first hominid to make and use tools, until we discovered that many primates make simple tools, such as stripping a stem of leaves to fish for termites. There has been a lot of debate about whether creativity and deliberate burial were unique to homo sapiens or could also be seen in Neanderthals and our common ancestor homo heidelbergensis. (My impression, from the research, is that all three species did this, but homo sapiens did it more extensively.) There have been a lot of debates about language, when and how it first emerged. We used to think Neanderthals didn't talk, but genetic evidence suggests they did (and may have even used a tonal language!). Now, the bigger question is whether homo erectus could talk, and how well.
This debate, for many people, involves an element of looking for the crucial step, the crucial point at which we became 'fully human'. In this way, it mirrors how many people think about severely disabled people – where is the line between a person who struggles with X and Y and someone who is not really a person anymore?
When homo sapiens first appeared, we shared our world with four other hominid species – Neanderthals, Denisovans, homo erectus and homo floresciensis (nicknamed 'hobbits'). We're not sure where Denisovans fit in (one suggestion is that they were a cousin to Neanderthals), but both us and Neanderthals descended from homo heidelbergensis, which descended from homo erectus. Homo floresciensis, who were tiny little guys, were another branch off of homo erectus.
All of these species lived fairly similar lives, making tools, eating a mix of meat and plant products, living in small, tight-knit social groups. Two of these species were actually close enough that we could produce fertile offspring, although probably with difficulty. (DNA research suggests that only daughters with Neanderthal fathers and homo sapiens mothers contributed to the small percentage of Neanderthal DNA in all non-African people. Their brothers were probably infertile, and the reverse crossing may not have been viable or may have only produced sterile offspring.)
I sometimes wonder how we might see ourselves and other species differently, if those other hominids had survived as separate populations. If, as in many fantasy stories, we shared our world with other species who are different and yet so similar, would we see them as people, or as talking animals? Would we even see such a divide? Would we still think of ourselves as so special and unique, if our closest relatives were still around to show us how non-unique we were? Or would we just move the line over a bit?
It used to be that we did not see personhood in such a black-and-white way. In the medieval era, a nobleman was more of a 'person' than his wife, and both of them were more 'people' than their servants were. Their servants, in turn, were more 'people' than a different ethnic group would be. Personhood was a spectrum. Over time, this idea fell out of favour, mainly because it led to some vicious prejudice.
But in some ways, our current black-and-white divide isn't that good a concept to replace it with. I don't think the same ethical standards apply to me as to my cat. If we saw all species as having the same moral rights, then my cat would be no different from Jeffrey Dahmer – both of them killed and ate other living creatures, not because they had to do so to survive, but because they enjoyed it. But I think a cat killing and eating mice for fun is very different from a human killing and eating another human for fun.
But where it gets messy is when the difference is less clear. If we didn't see Neanderthals as people, how would we see their children? How much Neanderthal ancestry would you need, before you weren't considered a person? (Incidentally, I have 3% Neanderthal ancestry, which is the same percentage I'd have if I had one great-great-great grandparent who was a Neanderthal.)
Well, let's say we did see Neanderthals as people. What about homo erectus? They had an average brain size about 2/3rds of our own. There's no evidence that they buried their dead, or showed any sign of imagination. There's a lot of debate about whether they used language or not. If they did use language, they'd have conveyed much simpler ideas, and may have had a simpler language structure. And yet they made tools and may have used fire to cook their food. And they loved their families, cared for the sick, injured or disabled, and worked together to achieve common goals.
It would be so interesting, getting to know a homo erectus. But for many people, it would probably also be quite threatening. They were so similar to us, but at the same time, they were so different. In speciation terms, there is no evidence that we successfully interbred with them – either we couldn't interbreed at all, or all offspring that resulted were sterile. But a homo sapiens and a homo erectus could certainly become friends, if both were open to the possibility. What kind of friendship would that have been?