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 Tree of Life (below) shows phylogenetic relationships among fifteen extant major phyla of "animals". All living forms appear as contemporaneous "tips". The phylogenetic 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 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. Note that in each case, the alternative branch is also defined by particular features.The exceptions are the Parazoa (absence of true tissues) and Acoelomates (absence of body cavities).

    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, such that the various branches are allowed to rotate freely.

    HOMEWORK: The Acoelomata lack body cavities: Redwood trees also lack body cavities. Does that mean these trees are Aceolomates? Why or why not?
                            The Tree includes only "Animals", from the Greek "anima" meaning "soul", which is the same root as "animated", capable of movement. The alternative is "Plants", which are "planted" and immobile (thus without 'souls'). It was argued at one time whether Porifera were Animals or Plants. Why? What is the basis of the modern terminology?