The Expanded Self: Society as Self 261“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityproducts that the cells can take in and digest. But when the concentra-
tion of sucrose in the medium is low, forming an aggregate gives them
a better chance to take up the split products than as single cells, simply
because of proximity to the useful nutrient.^5
Perhaps the most spectacular social behavior of single cells is seen
in the slime mold Dictyostelium discoidium. The life cycle of these tiny
amoeba alternates between single and communal existence, and provides
the first evidence of division of labor — differentiation. (For details, see
Fig. 4.3 in Chapter 4: The Microbial Self.) When nutrients are abundant,
they behave as individuals; when food is scarce, they aggregate and turn
into a slug, which literally “walks away” in search of “greener pasture.”
In the new environment the cells differentiate into a fruiting body con-
sisting of a stalk and spores, each with a unique function: the spores are
released and are capable of germinating into single cells for the contin-
uation of the species; the stalk cells support the spores and help in their
dispersion, while sacrificing their privilege of infinite propagation. Slime
molds straddle the immortal unicellular life and the limited lifespan of a
metazoan. The reproductive altruism of slime mold stalk cells serves as
a prototype of differentiation in higher organisms.
Endosymbiosis of two distinct single cells is equivalent to merging
two selves into one. Today, the most plausible explanation of the ori-
gins of certain organelles (the enclosed structures within a eukaryote)
is the engulfment of one free-living organism by another. The plastids
or chloroplasts of plant cells are believed to have come from cyanobac-
teria (bacteria capable of photosynthesis), whereas the mitochondria
in all eukaryotes are believed to have descended from proteobacteria.^6
In fact, vestigial DNAs are still present in chloroplasts and mitochondria,
betraying their foreign origin. Nonetheless, most genes in the endosym-
bionts today have long been transferred to, and integrated with, the
host’s nuclear genome, from where the protein products are allowed to
go back to the organelles to carry out their native functions — photo-
synthesis in chloroplasts and oxidative phosphorylation in mitochondria.
Historically, the functions added by the symbionts to the hosts enlarged