MadSci Network: Evolution

Re: how many generations have passed since Lucy?

Date: Fri May 1 09:01:25 2009
Posted By: Thomas M. Greiner, Assistant Professor of Anatomy
Area of science: Evolution
ID: 1239905901.Ev

How many generations since Lucy?

Lucy is the name given to a fossil human ancestor that has been dated to about 3.2 million years ago. Asking how many generations have occurred since that time seems like a simple problem. Itís just 3.2 million divided by the length of a generation. Unfortunately, thatís where the simplicity ends.

In your question you stated that you could not find a standard definition for the length of a generation. Thatís not surprising, since there really isnít one. You are aware that there can be as much as 35 years between the birth of a first child and a last child (not usually, but there can be). So, you might want to go for an average, but itís still pretty complicated.

Generation length is a perfect example of the combined influence of biology and culture that defines the human species. Age at first marriage is frequently used as surrogate measure here. People certainly have children outside of marriage, but marriage records have been kept for hundreds of years. So at least we know about that. It seems that the average age of first marriage for women has changed quite a bit in our society, from as low as 19 in fairly recent years to as high as 27 in the 1800s. Therefore, even using age at marriage as our basis for estimating generation length, weíve got a lot of variation. But culture can still creep in to determine how long a woman is reproductively active. In some cultures, women stop having children of their own after the birth of their first grandchild (known as cultural menopause). This can occur even though the woman in question is still relatively young (early 30s). In our culture, some women of that age have not yet had their first child. Cultural differences have a great influence in determining generation length, and as cultures change those determinations will change. This makes it very difficult to calculate an average generation length for humans.

But letís assume that we could eliminate the behavioral aspect of culture. That would leave only biological potential. Right? Wrong! Women enter puberty (a seemingly biological event) at different ages around the world. Girls in our culture can start puberty as young as 10 years old; 12 year old girls can, and do, get pregnant. This young reproductive age has not always been true, and girls in other cultures may not reach sexual maturity (menarche) until they are 18 years old. Therefore, we cannot determine a ďcorrectĒ physiological basis for a generation length.

Want another complication? There is no way to know if the generation length of modern humans is the same as that of past humans. Going back to Lucy is going back through several species shifts. Generation length, and the milestones of maturity, has almost certainly changed during the last 3.2 million years. Unfortunately, there is no way to know how, or when, these changes occurred. Therefore, the simple equation I started with simply will not work. Nonetheless, itís always fun to think about these things. When I was a student, the number 25 was always given as the average length of a human generation. I have no idea where that number came from, and as I hope I have explained it is almost certainly poorly derived. But, if we assume it is correct for the moment, that would give us about 128,000 generations between Lucy and us.

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