what is a common ancestor

A common ancestor is an ancestral group of organisms that is shared by multiple lineages. It is a concept in evolutionary biology that is applicable when one species is the ancestor of two or more species later in time. For example, the common ancestors of two biological siblings include their parents and grandparents; the common ancestors of a coyote and a wolf include the first canine and the first mammal. The more recent the ancestral population two species have in common, the more closely related they are. The most recent common ancestor of all currently living organisms is the last universal ancestor, which lived about 3.9 billion years ago. Similarly, shared nucleotide sequences, especially where these are apparently neutral such as the positioning of introns and pseudogenes, provide strong evidence of common ancestry. A clade or monophyletic group is a natural taxon in which all members are derived from a unique common ancestor with respect to the entire tree, have inherited.