Harnessing as a process similar to domestication. You can sometimes harness a wild animal and put it to use without domesticating it. Harnessing is often a prelude to domestication. As an example of the process, Mark claims that dogs have been domesticated, while cats have been harnessed. I see more harnessed dogs than harnessed cats, but Mark observes that saucers of milk count as a type of harness - in the sense in which he is using the term..
Mark applies the concept of harnessing to cultural evolution - claiming that humans have mostly been harnessed rather than domesticated. Many say that humans have been domesticated by cultural institutions. However, Mark is probably right that these are often cases of harnessing - rather than domestication.
I think that "harnessing" is useful terminology. It is also an important idea for students of cultural evolution. Humans are harnessed in many cases, just as Mark claims. Kudos to Mark for figuring this issue out and sharing his conclusions.