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 inferred to have descended. The most recent common ancestor of a higher group is generally thought to have been a species. We can also use this term when talking about the ancestry of groups of genes (haplotypes), not just whole organisms.

For groups of people, we sometimes try to figure out their ancestry by looking at family trees, called pedigrees. But for large groups, it's usually impossible to know the exact MRCA. Instead, scientists can often estimate when the MRCA lived. They do this using DNA tests and known mutation rates, or by using mathematical models and computer programs.

In organisms that reproduce sexually, we can talk about the matrilineal MRCA (only through the mothers) and the patrilineal MRCA (only through the fathers). For humans, the matrilineal MRCA is known as "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 either Mitochondrial Eve or Y-chromosomal Adam, who are estimated to have lived about 200,000 years ago.

When we study groups of species or higher categories, we cannot observe their ancestors directly. Instead, we make inferences based on patterns we see in living organisms 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)

The idea of finding the most recent common ancestor helps us understand how different species are related. Scientists work on a big project called the "tree of life" to show these connections. They figure out when different groups of animals last shared a common ancestor. For example, the ancestor shared by all cats and dogs lived about 42 million years ago.

In books about human history, authors describe the many ancestors we share with other animals. They list the common ancestors between humans and chimpanzees, then between humans and gorillas, and so on, all the way back to the very first ancestor we share with bacteria.

MRCA of a population identified by a single genetic marker

Main article: Coalescent theory

We can also look at the history of single genes or small groups of genes, called haplotypes, instead of whole living things. Coalescent theory explains how these genes change and pass down through families over time.

Genes are passed from one group of living things to the next either exactly the same or with small changes. Unlike whole living things, which can have two parents because of how they reproduce, a single gene always has just one parent from the past. This makes studying genes a bit simpler, as 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 a long time ago.

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 types of ancestors we share all lived at different times in the past. These times are worked out in different ways depending on what kind of ancestor we are looking at.

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

Some scientists think the most recent common ancestor of all humans might have lived as recently as 2,000 years ago, possibly in East Asia. This is because people from very isolated places like Australia and the Americas might have come from this area. It is important to remember that this ancestor did not pass all their genes to everyone alive today. Through having children, each person only passes 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 ancestor 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.