Like other complex organisms, people vary somewhat in size and shape, skin color, body proportions, body hair, facial features, muscle strength, handedness, and so on. But these differences are minor compared to the overall similarity of all humans, as demonstrated by the fact that people from anywhere in the world can reproduce with each other and donate blood or organs to one another. Humans are indeed a single species. Furthermore, as great as cultural differences between groups of people seem to be, people's complex languages, technologies, and arts unite them as a species distinct from others.
For Grades: 9-12
Learning Goal 1
The similarity of humans in their cell chemistry and DNA sequences reinforces the idea that all humans are part of a single species.
Learning Goal 3
Fossil and molecular evidence supports the idea that human beings evolved from earlier species.