Symbiosis represented a significant revolution in evolutionary theory because it introduced a new operation into the fundamentals of evolutionary theory: merging or joining. If splitting looks like this:
The pioneers of symbiosis were mostly Russian. Though they discovered symbiosis early in the 20th century, their theories were ignored in the west until the 1960s, when it was discovered that mitochondria contained their own DNA lineages. Over the next 30 years, evidence accumulated that unrelated organisms could fuse together permanently, generating new kinds of recombination operation distinct from sex. Gaining or losing symbionts could be profound and dramatic evolutionary events - sometimes triggering speciation. The phenomenon was not confined to bacteria, but affected organisms of all sizes. We now know that eucaryotic cells are unions of many free living ancestors, that viruses carry DNA between all kinds of different species, and that around 10% of the human genome shows signs of origins outside our own species. Horizontal gene transfer has turned the tree of life into a web. Multi-cellular organisms are now widely viewed as being menageries.
In modern times, the evolutionary significance of symbiotic unions is familiar to us, so it is sometimes hard to appreciate the scale of the revolution the idea represented, and the resistance that it faced at the time. Evolution was generally though of as being gradual - yet in symbiogenesis, entire genomes could join forces in an evolutionary instant. This idea violated established dogma and was widely rejected and ridiculed for over 50 years. An avalanche of facts and data eventually vindicated the idea. At the time, this was the the biggest revolution in evolutionary theory since Darwin - eclipsing the discovery of DNA, kin selection and Mendelian inheritance, in my humble opinion.
In cultural evolution, the story went rather differently. In 1975, Ted Cloak pioneered a symbiosis-based version of cultural evolution, in which human hosts cultivated cultural symbionts inside their brains. Richard Dawkins went on to popularize the idea in the 1976 book The Selfish Gene. These cultural "memes" were sometimes characterized as being "viruses of the mind". However, in academia, symbiosis was very slow to catch on. Many academics didn't seem to cotton on to the fundamental concept of a cultural organism - instead opting to model culture as an aspect of the host's phenotype. This was a bit like modeling smallpox without the concept of the smallpox virus - and instead talking about horizontal and oblique transmission of the smallpox rash phenotype. Many of the researchers in the field are still laboring under the resulting hangover. Symbiosis enthusiasts were one tribe of biologists and cultural evolution enthusiasts were another. The intersection included Ted Cloak, Richard Dawkins and Ben Cullen - but only a few others. What this meant in practice was that concepts such as parasitism, mutualism, domestication, parental care and arms races tended to get neglected by most students of cultural evolution.
The slow uptake of symbiology among cultural evolution researchers is an aspect of cultural evolution's scientific lag. Most anthropologists long ago demonized evolution and expelled it from their halls as a dastardly idea that they wanted nothing to do with. The important topic of cultural evolution subsequently suffered from lack of attention and funding - and progress has been slow as a result. It wasn't until the 1980s that symbiology gained acceptance among mainstream evolutionary biologists. There's clearly still a long way to go before most students of cultural evolution start applying the idea properly.