There are no two-celled life forms intermediate between unicellular and
multicellular life, demonstrating that the intermediate stage is not
viable.
Source:
Brown, Walt. 1995. In the Beginning: Compelling evidence for creation and
the flood. Phoenix, AZ: Center for Scientific Creation, p. 9.
http://www.creationscience.com/
Response:
The intermediate stage between one-celled and multicelled life need
not have been two-celled. The first requirement is for signals between
cells, which is necessary if cells are to cooperate in division of
labor to break down a food source. Many bacteria utilize a variety of
different signals. The evolution of a signal for cooperative swarming
has been observed in one bacterium (Velicer and Yu 2003).
The transition to multicellularity has been studied in experiments with
Pseudomonas fluorescens, which showed that "transitions to higher
orders of complexity are readily achievable" (Rainey and Rainey 2003,
72). Choanoflagellates, which are unicellular and colonial organisms
related to multicelled animals, express several proteins similar to
those used in cell interactions, showing that such proteins could arise
in single-celled animals and be co-opted for multicellular development
(King et al. 2003).
Desmidoideae is a class of conjugating green algae, phylum Gamophyta.
Most desmids form pairs of cells whose cytoplasms are joined at an
isthmus (Margulis and Schwartz 1982, 100). The bacterium Neisseria
also tends to form two-celled arrangements. As noted above, this may
not be relevant to the evolution of multicellularity.
References:
King, Nicole, Christopher T. Hittinger and Sean B. Carroll. 2003.
Evolution of key cell signaling and adhesion protein families predates
animal origins. Science 301: 361-363.
Margulis, Lynn and Karlene V. Schwartz. 1982. Five Kingdoms
San Francisco: W. H. Freeman.
Rainey, Paul B. and Katrina Rainey. 2003. Evolution of cooperation and
conflict in experimental bacterial populations. Nature 425: 72-74.
Velicer, Gregory J. and Yuen-tsu N. Yu. 2003. Evolution of novel
cooperative swarming in the bacterium Myxococcus xanthus.
Nature 425: 75-78.
Further Reading:
Bonner, John Tyler. 2000. First Signals: The evolution of multicellular
development. Princeton University Press.
Cavalier-Smith, Tom. 2002. The phagotrophic origin of eukaryotes and
phylogenetic classification of Protozoa. International Journal of
Systematic and Evolutionary Microbiology 52: 297-354. (technical)
