Brooke Johnson

One of the challenges of studying Earth Sciences is trying to get people to understand the concept of deep time. To colleagues in archaeology 10,000 years is a long time, but the Earth is around 4.5 billion years old, to a geologist 10,000 years is just a slice of sediment the width of your finger. To a geologist, a million years is a blink of an eye and living things are flickering shades, gone and then forgotten, save for their fossils. To those outside Earth Sciences, these time scales are abstract to the point of being incomprehensible. It’s little wonder most people with geological time scales when presented by such a yawning temporal chasm. Deep time could swallow you, your whole life and the entire history of the human race and still be mostly empty. If you want to explore such distant worlds, then even a simple map might be useful.

Thinking about deep time in terms of human lives helps provide such a map. If by age 50 a modern human has at least one child and one grandchild then we can calculate back through generations to see how many relatives separate us from key points in the deep past. The last mammoths died around 33 granddads ago, while the Romans arrived in Britain ~40 granddads ago. The great pyramids of Giza where built 51 granddads ago and the last glacial period ended just 200 granddads ago. So now we have an idea of how few granddads separate us from well known “ancient events”, But what about further back?

Lucy is perhaps the most famous hominin fossil known to the public and certainly to the science community. Discovered in 1974 in Ethiopia by Donald Johanson, she quickly became a celebrity and allowed huge leaps to be made in studies of human evolution. Lucy was an Australopithecus afarensis, one of the many steps in human evolution on the way to ourselves, and lived about 3 million years ago. Modern humans are undoubtedly related to Lucy and her kind but how far removed are we in terms of human lives? In this case it’s around 60,000 granddads (e.g. 60,000 x 50 years), at least roughly speaking, this is not a high fidelity method by any means. That’s not a large number, it illustrates the ephemerality of human lives and brings the past within touching distance. All that separates you from the virgin Pleistocene savannah and the first faltering steps that would lead to modern humans is a number less than the population of a small town.

Thinking in terms of granddads, or generations, can be useful for looking at timescales for evolutionary processes as well as historical or environmental changes. When looking into the evolution of complex eyes (no pun intended), computer models show that it would only take 30,000 generations to go from photosensitive metabolic by-products to the camera style eyes prevalent today. While 30,000 granddads takes us half way back to Lucy, for invertebrates like worms and jellyfish, a generation may be months or even days. The advantages of sight in an eyeless world would allow it to be aggressively selected for by evolution. Incidentally both predatory polychaete worms and certain jellyfish have complex eyes and the associated intelligence and hunting skills to make use of them.

We are the latest iterations in a long lineage of survivors that crawled up through history one ancestor at a time. Knowing how good the Earth is at killing things en masse e.g. the dinosaur extinction, this is an achievement to be celebrated. Yet since the industrial revolution, a mere 4 granddads, human activity threatens the environment upon which we all depend.

How many granddads will it take to learn from and rectify our mistakes?

Let’s hope it’s not too many.


Nilsson, D.E. Pelger, S. A Pessimistic Estimate of the Time Required for an Eye to Evolve, Proceedings of the Royal Society B 22 April 1994.DOI: 10.1098/rspb.1994.0048.
The Science of Discworld III: Darwin’s Watch, Ian Stewart, Jack Cohen, and Terry Pratchett, Random House, 2005
Evolving the Alien, Ian Stewart and Jack Cohen, Ebury Publishing, 2002