We live in a culture where 40% of people don’t believe the world is more than 6,000 years old. And yet how can an intelligent being hold such beliefs when faced with a 13,000-year-old eucalyptus tree or an 80,000-year-old aspen? But even when we embrace science completely, one of the most baffling aspects of the timeline of evolution — for creatures as dependent on categories as we are to make sense of the world — is its incremental progress largely devoid of clear markers denoting when one primitive species ends and its evolved successor begins.

Inspired by The Magic of Reality: How We Know What’s Really True — Richard Dawkins’s children’s book seeking to replace myth with science — PBS’s Joe Hanson offers a concise and elegant explanation of why there was no “first human.” Tracing any one person’s family tree — yes, yours, as well as mine — back 185 million generations takes us not to another human but to a fish, which begs the question of where the human species “began”:

You can never pinpoint the exact moment when a species came to be — because it never did. It’s just like how you used to be a baby and now you’re older, but there was no single day when you went to bed young and woke up old… Evolution happens like a movie, with frames moving by both quickly and gradually, and we often can’t see the change while it’s occurring. Every time we find a fossil, it’s a snapshot back in time, often with thousands of frames missing in between, and we’re forced to reconstruct the whole film. Life is what happens in between the snapshots.

For a closer look at The Magic of Reality, go here, then see more of Henson’s terrific science illuminators, like the science of why we kiss, the mathematical odds of finding your soulmate, and why we can consider the avocado a curious ghost of evolution.