There is no single "line" that cleanly separates one species from another in the natural world, as evolution is a gradual process. This doesn't imply that you can't define categories, and if you dig up human skeletal remains from a hundred years ago, you can categorize them based on a combination of genetic, anatomical, and behavioral characteristics. The point is that these categories are not an ethical dividing line to determine whether someone deserves their negative rights.
There is a continuum in all traits that define what it means to be human, making the difference between species gradual and not absolute.
The concept of the species boundary becomes less clear when we consider the implications of time travel. Let's envision a scenario where you take your mother's hand, who then takes her mother's hand, and so on, tracing back through the generations and spanning thousands and millions of years along your family tree. Simultaneously, in Africa, a chimpanzee does the same by taking its mother's hand and leading it back in time to the common primordial mother. In this thought experiment, a long chain is formed, connecting humans and primates over a distance of approximately 1000 kilometers.
What emerges from this perspective is the recognition that you and the chimpanzee are distant relatives, separated by vast spans of time. You, as a human, and the chimpanzee metaphorically shake hands across time. If morality were strictly confined to the boundaries of species, then somewhere along this chain, there would have to be a mother and a daughter where one is still considered a human being and the other a non-human animal. In such a scenario, morality would have to be applied equally to both individuals, despite one being human and the other a non-human animal. However, arguing that morality ends at the species line would imply that morality applies to one and not the other.
If one's moral boundaries are constricted to the human species, then they must also accept that if we were to discover a group of individuals from Cambodia who were physically and cognitively indistinguishable from us but wouldn’t fall into the category of human, constituting a distinct species with almost identical capabilities, anyone using this criterion to determine moral worth would have to acknowledge that any atrocities committed against these individuals would be considered morally acceptable. If a DNA test were to categorize past victims of atrocities like genocides as something other than human, the individual responsible for defining the moral boundaries of consideration would assign less value to actions such as torturing, enslaving, dismembering, or even consuming these individuals for pleasure, viewing them as morally neutral or less considerable than if these events had happened to the human species. However, this perspective obviously doesn't align with the values of anyone who isn't a psychopath.
Imagine a situation in which a person causes significant harm to others while being fully conscious and aware of their actions. During these actions, they genuinely believed that the victims did not belong to the Homo sapiens species. In this scenario, we would not consider their misguided belief as a justification, as we might if someone suffering from a mental illness didn't grasp the consequences of their actions. This is because the wrongfulness of torture does not depend on how we classify species.