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 the Moneren at bottom,  thence ascending to the 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 extant 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.

    The left-to-right arrangement of phyla reflects the notion or progression from 'simple' to 'complex' creatures, which is true to the extent that successive evolution of (1) -> (2) -> (3) -> (4) results in more complicated animals. However, the Tree may also be thought of as a "Mobile", if picked up by the "ancestral protist" base so that the various branches are allowed to rotate freely.