One of the better examples I’ve yet found that conveys the concept of evolution comes from the 2002 NOVA documentary “Search for the First Human.” The main focus of the video is the species Orrorin tugenensis and it’s possible place in our family tree six million years ago. However, two particular segments stand out to me, and I think do a pretty good job of conveying the idea of evolution to students.

The first is a CGI animation of a person standing in a chain of ancestors, with one parent placed directly behind its progeny, and stretching back into the distant past. About 12 and a half minutes into the video, the narrator informs us that the first modern human appeared around 100,000 years ago, and would stand roughly 3 miles behind us. We now think the first modern humans appeared earlier than this (the video is a bit dated), around 200,000 years ago, so we can double that estimate to about 6 miles. If we keep that ratio, our ancestor at the two million year mark would stand about 60 miles behind us; at six million years, about 180 miles, etc. I suppose the actual ‘distance’ depends on how much space we put between parent and offspring, but to me the actual length of the chain is less important than the fact that it’s a thought-provoking way to visualize ancestry.

A chain of ancestors stretching back in time

There are a few other, unspoken, pieces to this segment.

(1) It conveys how the existence of each individual in the chain is contingent upon the previous link’s ability to survive and reproduce. Should one of the links in the chain be broken, all subsequent ones simply would not exist. It is staggering to consider how many chances there were for our own chains to be broken. But it wasn’t. So good for you.

(2) I think it also shows how few of our direct ancestors we actually know, and how quickly our own individual biological ancestry can become clouded in mystery. Even for those of us with the best documented genealogies (such as royalty), the picture becomes clouded after only a handful of generations. This becomes even more complicated when we remember that each individual in the chain has two parents standing behind them, not one (obviously). If we go back only ten generations (roughly 250 years), each of us has over 1,000 direct ‘great-grandparents.’ How many of them do we actually know?   

(3) By illustrating each link in the chain, the idea of gradual evolution becomes easier to comprehend. Genetically, no child would be exactly identical to either of its parents, but no parent would give birth to an offspring belonging to a different species. Instead, speciation is a gradual phenomenon. 

Diverging branches on the tree of life

The other part of the video occurs only a few moments later, around the 15 minute mark. Here we see that an understanding of a population perspective is essential, and that looking only at one individual’s chain of ancestors cannot give us the whole story. The screen capture above contains a lot of information. Each tiny dot represents a single, genetically unique, individual in a population. The left part of the screen represents the past, while the right is our present. The diverging branches illustrate separate evolutionary paths, or speciation events, with some species becoming extinct and unable to make it to the present. Finally, the thickness of the branches represents the size of a population, thinner at some points, thicker at others. If we go back in time far enough, all branches would coalesce at some point, not only for different species of hominids, but for all species.

One picture really can be worth a thousand words.