The cells of complex organisms are themselves pretty complex. They contain within them many fine-tuned structures with precise functions that support the whole cell and ultimately the whole organism. How exactly such compartmentalised complexity came about was a question that scientists struggled for decades to answer; a gradual evolutionary approach of step by step improvements didn’t really seem to fit. Proponents of ‘intelligent design’ described these biochemical machines as irreducibly complex. In the 1960′s, Lynn Margulis had suggested a bold and controversial idea; eukaryotic cell organelles were once free living forms that were engulfed and recruited into symbiotic relationships. The hypothesis was widely criticised at the time but support grew as the evidence mounted and her idea of endosymbiosis finally reached orthodoxy once scientists established that mitochondria and chloroplasts have their own DNA, independent from the DNA in the cell nucleus. In addition there are strong similarities, both morphological and genetic, between these cell bound organelles and free living forms that still exist. Chloroplasts are homologous to cyanobacteria whilst mitochondria are homologous to purple bacteria.

Having achieved enormous recognition for her work on the endosymbiotic origin of eukaryotic cells Lynn Margulis went on to extend her ideas to wider forms of complex life, even believing that symbiosis is the main driving force of evolution. In recent years these highly controversial views have seen her marginalised once again and while I would put myself amongst the majority who think she has taken her hypotheses a step too far it is true that evolution occurs through several means, involving mechanisms that Darwin could not have envisaged.

It has been known for a while that some sea slugs can photosynthesise using chloroplasts extracted from their algal food but recently scientists have established that algal genes have found their way into the slug genome, a process called horizontal gene transfer. The resulting organism has been dubbed half-animal, half-plant due it’s ability to live off sunlight and the very leaf like body it has evolved to maximise it’s sunlight gathering potential. A compelling 2008 paper* concluded:

"Molecular evidence is presented supporting eukaryotic multicellular
interdomain HGT (including into the germline) using a
mollusc model and expression of an essential algal nuclear gene
required for photosynthesis."
"In light of these findings, the prospect of natural HGT
taking place between distantly related organisms, especially with
any physical contact, must be considered formally possible. This
is especially true in the context of genetically modified organisms.
The implications for evolution and speciation through
acquisition of foreign parts and selected genes to produce new
lineages, as proposed by Margulis (2), are heightened by this
unusual photosynthetic mollusc."

* Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica: Mary E. Rumphoa,1, Jared M. Worfula, Jungho Leeb, Krishna Kannana, Mary S. Tylerc, Debashish Bhattacharyad, Ahmed Moustafad, and James R. Manharte