The process of inheritance, the mechanism of inheritance, have always been important to how we understand the process of evolution. Even though Darwin didn't have a fully articulated idea of how that inheritance occurred, it was an essential part of his model for evolution. Now in the beginning of the 20th century, as Mendel's ideas began to filter in devolutionary theory, the actual mechanism, genetic inheritance, became increasingly important for how we understand evolution. But over the last 25 years, genetics had become a critical tool for understanding the specifics of human evolution, both the timing of when certain events happened in our evolutionary past, and the process by which they happened had become amenable to modern genetic techniques. Now, essential to understanding actually how to use genetic variation to interpret the past is thinking about how genetic variation formed. If we look at a living population today, if we look at a set of individuals within that population, they encompass a certain range of genetic variation. They each contain a certain number of mutations that are unique to themselves. However, we know that those mutations, at some point in the past, come from a common ancestor, so that any two individuals, at some point, share an ancestor in the past. Now if we look at any two individuals within that population, we know that at some point in the past, although they're different now, they share a common ancestor, or they share a common genetic variation. And we can estimate with what probability they shared that ancestor one step in the past. Now more likely, they don't share an ancestor that one step in the past, and we can estimate that probability as well. But if we carry this forward through a process that we refer to as coalescent, at some point, all of these individuals are going to share a common ancestor in the past. So all of the genetic variation we see in living humans today, at some point, coalesces to a single version of that variation in the past. And it turns out we can, with some degree of precision, estimate probabilistically how long in the past that took place. Assuming we know the pace at which mutations arise within a population, and the probability that any two individuals share an ancestor one generation in the past, we can estimate how many generations in the past, all of these individuals sampled within this population share a common ancestor. And we can extend this process, actually, from a population to different populations, and estimate as a species, when do we share a common ancestor in the past. And it turns out we can do this across a wide variety of genetic systems. Following this section, we'll look at, actually, a specific example from mitochondrial DNA, just one element of our genetic variation, and how that was used in the late 1980s to estimate when we share a common ancestor in the past, when we all share a common ancestor in the past. But it turns out these methods have developed even more broadly today to give us a very refined view of population history and species history by looking at patterns of genetic variation. Living humans, the genetic variation we all share, is a product of the evolutionary events that have led to us being here today. Therefore, looking at the genetic variation of living human populations gives us a signal or a sign into our evolutionary past. We can reconstruct that with a certain probability and a certain theoretical framework, how long in the past that took place. So we can use that to time the development of certain evolutionary events. Now we can also use genetics in other ways these days as well. We can actually look at the evolution of specific genes and figure out when specific genes have changed in the past to try and get an estimate of when certain selection events have occurred in the past as well. Something we'll talk about later in the course. So by looking at the variation we see in living populations today and modeling that variation back to a common ancestor in the past, we can reconstruct when and where major evolutionary events in human history took place.