Recapitulation
Ernst Haeckel (1834-1919), the main evolutionist in Germany during his time, was one of the first scientists to propose a model for the development of multicellular organisms from unicellular ancestors. He proposed that the development of an animal's embryo today reflects its evolution. This concept, the theory of recapitulation, states that organisms go through a series of stages during their embryonic development that resemble the adult forms of their ancestors. However, Von Baer's suggestion that young stages of development resemble young ancestral stages is more widely accepted.
Haeckel's theory is largely discredited today. Evolution is based on genetic change through mutations over time. Recapitulation requires both retention of the ancestral features and change. Just because structures appear the same does not mean that they were developed from the same ancestor.
As Michael Denton points out, homologous organs and structures may develop by radically different embryogenic routes, and "the evolutionary basis of homology is perhaps even more severely damaged by the discovery that apparently homologous structures are specified by quite different genes in different species."ii
The Gastraea Hypothesis
Ernst Haeckel also proposed a mechanism whereby unicellular organisms may have evolved to form multicellular, and eventually multi-layered organisms. This theory is known as the Gastraea Hypothesis. Today, the Planula Hypothesis, a variant of the Gastraea Hypothesis, is more popular, but the problems remain the same as for the Gastraea Hypothesis of Ernst Haeckel.
Haeckel proposed that multicellular organisms evolved from hypothetical unicellular organisms which he called Cytaea. Eventually these cells remained attached after cell division and a multicellular organism which he termed Moraea evolved. The Moraea gave rise to a jelly-filled hollow ball of cells, called Blastaea which developed an indentation on one side and thus gave rise to the Depaea. Through completion of the indentation, the Depaea gave rise to the Gastraea.
The Gastraea then underwent further differentiation. A third layer of cells developed between the original germ layers. He proposed that this layer, the mesoderm, arose through cellular migration from the outer ectoderm and inner endoderm, thus giving rise to triploblastic organisms (animals with three layers) which would then also have evolved bilateral symmetry after becoming bottom dwellers. Associated with the change in structure there would also have occurred cellular differentiation and specialization, thus giving rise to complex organisms where cells became arranged into organ systems.
For most of these proposed ancestral forms, analogous living forms are presented as evidence for the viability of such organisms. The Cytaea could have resembled living protozoa of the Class Mastigophora, the Moraea represents colonial protozoa such as Pandorina, the Blastaea in turn can be compared to colonial protozoa such as Volvox. The evolution of subsequent stages would have required some complex changes, and it is proposed that the modes of feeding and locomotion of the ancestral types would have affected further differentiation. The bottom dwelling triploblastic animals that developed bilateral symmetry could be compared to present day flatworms.
Now that we understand Haeckel's theory, let's see how it measures up in the biological worldi M.W. Strickberger, Evolution (Jones and Bartlett Publishers International, 1996).
ii Michael Denton, Evolution: A Theory in Crisis (London: The Hutchinson Publishing Group, 1985).
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