Ever seen other animals and wondered just how many years have passed since you shared a common ancestor with that chimp, or that bird? Well, wonder no more!
A sweet tool exists out there to help you – called Time Tree: The Timescale of Life. If you go far back enough in the history of humans, eventually we all lead to a single related population. All humans share a common ancestor (our great-great-great-great plus seven thousand more greats grandma) and we can trace our cousins’ and our family lineage to some common relative (like how 1/2 a percent of the world can trace their ancestry back to Genghis Khan). We can trace all of today’s living species to common ancestors that existed millions of years ago in a similar fashion. Time Tree is a tool that summarizes scientific literature to tell you the point in geologic time that a single lineage split into two – say, when the human and chimpanzee lineages split from a shared ancestor.
So you put in the names of two taxa:
And check out the results:
So the human and chimp lineages split approximately 6.4 million years ago. Now keep in mind that humans and chimps didn’t exist 6 million years ago – rather an ancestral species that gave rise to both humans and chimps dates from this time. Time Tree has scoured the scientific literature to find molecular time estimates (each of the black dots above is a different scientific study). Time Tree then averages the time estimates to give its final output. The differences in times for the studies has to do with the number of genes and the specific methodology the researchers used to calculate the divergence times.
Scientists use a combination of dated fossils and genetic data to estimate divergence times for species. Fossil ages are estimated by using radiometric dating, where measuring the ratio of unstable to stable isotopes in the fossil material allows scientists to determine how long the unstable isotopes have been decaying, and how long ago the fossil formed. Molecular clocks are a more recent technique for estimating the divergence of two lineages.
Simon Ho over at Scitable explains molecular clocks:
The molecular clock hypothesis states that DNA and protein sequences evolve at a rate that is relatively constant over time and among different organisms. A direct consequence of this constancy is that the genetic difference between any two species is proportional to the time since these species last shared a common ancestor. Therefore, if the molecular clock hypothesis holds true, this hypothesis serves as an extremely useful method for estimating evolutionary timescales. This is of particular value when studying organisms that have left few traces of their biological history in the fossil record, such as flatworms and viruses.
Even though the molecular clock hypothesis states that DNA evolves at a relatively constant rate, different genes can mutate at slightly different rates, which explains the range of time estimates from the scientific studies cited in Time Tree. Also, these data on divergence times are opportunistic. The 40 scientific studies cited in the human chimp lineage split did not all set out to determine only this split – most of the studies are asking different research questions that happen to use the human/chimp divergence time, say as a reference point in a study that is really about whales. This would change the focus of the data collection and analysis in a way that could give a wider range of time estimates. Time Tree doesn’t really account for this, but it is still a useful tool and the time estimates are fine for the purpose of ball-parking divergence times.
Let’s see what happens if we search for the divergence time between the evolutionary lineage that lead to humans and the one that led to man’s best friend the dog:
Or human and our city neighbor the pigeon:
Or us and the tsetse fly, a nasty biting fly:
What the heck, humans and flowers:
Seems pretty intuitive that between dogs, chimps, birds, flies and flowers we share a more recent common ancestor with chimps, then further back in time our ancestral lineage links up with dogs, then birds, then waaaay back in time to flies and flowers. We could keep going to bacteria and other weird stuff, or we could take a less human-centric approach, but I’ll let you investigate that. Check out Time Tree, and see how long ago we shared a common ancestor with any of our distant evolutionary cousins populating the Earth today.
Thanks to Tim Sosa for comments.