Most recent common ancestor

A most recent common ancestor (MRCA), also known as a last common ancestor (LCA) or concestor, is the most recent individual from which all organisms of a set are thought to have descended. The MRCA of a larger group is usually thought to have been a species. We can also use this idea for groups of genes (haplotypes), not just whole living things.

For groups of people, we sometimes look at family trees, called pedigrees, to understand their ancestry. But for very large groups, it's hard to know the exact MRCA. Scientists can often guess when the MRCA lived. They do this using DNA tests and known mutation rates, or by using math models and computer programs.

In living things that reproduce with two parents, we can talk about the matrilineal MRCA (only through mothers) and the patrilineal MRCA (only through fathers). For humans, the matrilineal MRCA is called "Mitochondrial Eve" and the patrilineal MRCA is called "Y-chromosomal Adam". We don't know exactly when the human MRCA lived, but it must be no older than Mitochondrial Eve or Y-chromosomal Adam, who are thought to have lived about 200,000 years ago.

When we study groups of species or larger categories, we cannot see their ancestors directly. Instead, we make guesses based on patterns in living things and fossils. The last universal common ancestor (LUCA) is the most recent common ancestor of all life on Earth today, and it is thought to have lived around 3.5 to 3.8 billion years ago, in the Paleoarchean era.

MRCA of different species

Further information: Chimpanzee–human last common ancestor, Last universal common ancestor, Phylogenetic tree, and Tree of life (biology)

Finding the most recent common ancestor helps us see how different species are related. Scientists work on a big project called the "tree of life" to show these links. They study when groups of animals last shared a common ancestor. For example, the ancestor shared by all cats and dogs lived long ago.

Books about human history talk about the ancestors we share with other animals. They name the common ancestors between humans and chimpanzees, then humans and gorillas, going back to the first ancestor we share with bacteria.

MRCA of a population identified by a single genetic marker

Main article: Coalescent theory

We can study the history of single genes or small groups of genes, called haplotypes, instead of whole living things. Coalescent theory helps us understand how these genes change and pass from one generation to the next.

Genes move from one group of living things to another either unchanged or with small changes. Unlike whole living things, which can have two parents, a single gene always has just one parent from the past. This makes studying genes easier because each gene has a single line of ancestors.

Patrilineal and matrilineal MRCA

Main articles: Mitochondrial Eve and Y-chromosomal Adam

Some genes come only from the mother or only from the father. These can help us learn about our family history.

One kind of gene, called mitochondrial DNA, is passed down only from mothers to all their children. Scientists use this to find Mitochondrial Eve, the most recent person everyone comes from on their mother’s side.

Another kind of gene, found only in males, is passed down from fathers to their sons. Scientists use this to find Y-chromosomal Adam, the most recent person everyone comes from on their father’s side.

Time to MRCA estimates

Different ancestors we share lived at different times in the past. We find these times in different ways, depending on the ancestor.

When looking at special genes from only mothers or fathers, scientists use DNA tests and known gene change rates to guess when these ancestors lived. But for the most recent ancestor of all humans, we can't use genes alone because most genes are lost after a few hundred years. Instead, scientists use math models and computer programs to estimate this time.

Some scientists think this ancestor might have lived as recently as 2,000 years ago, possibly in East Asia. This is because people from isolated places like Australia and the Americas might have come from this area. Remember, this ancestor did not pass all their genes to everyone today. Each person passes only half of their genes to the next generation, so the genes from one ancestor become very small over time.

Identical ancestors point

Main article: Identical ancestors point

The most recent common ancestor (MRCA) is the last person from whom everyone in a group is descended. This person might have lived at the same time as others who were also ancestors to some people but not everyone.

The identical ancestors point is an earlier time in history, before the MRCA, when all people share the same ancestors. Even after this point, people can still be more closely related to each other because of how family trees sometimes overlap.