The horizontal axis of the cladogram depicted below is a timeline that extends from 100,000 years ago to the present; the vertical axis represents nothing in particular. The labeled branch points on the tree (V-Z) represent various common ancestors. Let's say that only since 50,000 years ago has there been enough variation between the lineages depicted here to separate them into distinct species, and only the tips of the lineages on this tree represent distinct species.In Darwin's tree of life, each fork in the tree represents ________.
a. morphologic gaps in the fossil record
b. groups of living organisms
c. groups of extinct organisms
d. the most recent common ancestor of the subsequent branches

Charles Darwin used the concept of a tree of life to illustrate and support his theory of evolution. In Chapter V of his famous book On the Origin of Species (1859), Darwin presented a diagram of a theoretical tree of life. In this tree, distinct species are represented at the end of the branches and each fork in the tree represents the most recent common ancestor of the subsequent branches. Attached is an image of the original diagram as reference.