The Tree of Life

    The German biologist Ernst Haeckel (1834 - 1919) was a noted scientific illustrator and the first great popularizer of Darwin's Theory of Evolution. This Tree of Life from his "General morphology of organisms ... by the descent theory reformed by Charles Darwin" (1866) traced the origin of all life to Moneren, and shows the shared ancestry of Humans among the Great Apes. Note the position of Menschen ("Humans") at the very top of the tree, among the Affen ("apes"). This arrangement of living forms reflects a judgement that those at the bottom are "primitive" and those at the top are "advanced." It also suggests that evolution is linear, proceeding up the main trunk to humans as the highest form of life.

    A modern phylogenetic Tree of Life (below) shows relationships among fifteen major phyla. All forms appear at the "top" of the tree, as all living forms are contemporaneous in time. The emphasis is on evolution of major characters that allow recognition of related groups on shared branches. The vertical axis is Time, which runs at the same rate for all organisms. Significant evolutionary events in the temporal order in which they occurred include (1) differentiation of distinct tissue types in Eumetazoa, (2) development of bilateral symmetry in Bilateria, (3) development of body cavities in coelomates, and (4) evolution of deuterostomic development in the common ancestor of Echinodermata and Chordata.