Naturally he won't be able to provide documents as this tradition pre-dates writing.

However, he's not exactly, I think, entirely right. While a big part of the REASON marriage, or some kind of analogue of it, got started was due to agricultural property inheritance, we know that religion pre-dated agriculture as plenty of evidence for it is found in pre-agricultural anthropology. It seems likely that in those early cultures, religion and the 'marriage' ritual (which would have looked nothing like a Christian marriage of course) were probably pretty closely tied.

The cultural goal was to solidify and clarify inheritance, yes. However it is also worth noting that 'marriage' in some form may even somewhat pre-date agricultural settlement as quite a few nomadic cultures had an analogue. The Inuit do/did for instance, as did the plains indians and both were nomadic. However both were in contact with agricultural peoples and both have concepts of ownership so the tradition may be borrowed, or may be tied to the idea of ownership. I don't think anyone is entirely sure (and the reasons may not even be universal cross cultures).



Okay - just to be clear here - 'marriage' has not been 'defined for pretty much existence'. Not even close. Human beings have been on this planet for around a couple of million years. We don't know precisely how long marriage has been around, but what indications we do have don't show any evidence of it for much of that time. Civilization is only some ten to twenty thousand years old. (The step pyramid at Sahkarah is circa 12,000 and is not a bad rough marker for the start of 'civilization' as we might define it). I believe we can find records of some forms of familial bonding going back to the Sumerians but I don't know of any older than that. There are records that predate that time (not documentation, obviously as writing wasn't invented yet) but none of those that I know of give us any evidence of formal familial bonding.

Look at records throughout history. In the written archive, we see marriage records going back thousands of years. Note that these 'marriages' are not always by any means 'one man and one woman' that's a fairly modern concept that people are trying to apply in a revisionist way. Lots of ancient cultures practiced polygamy, including the pre-Christian Jews and a great many of their contemporaries. Polyandry and group marriages were not as common due to the biological impact of how such things affected inheritance. Also, due to warfare, there was frequently a shortage of men and a need for more babies to grow into soldiers. In fact Athens, which normally practiced non-plural marriage, briefly made polygamy not only legal but in some cases mandatory in order to breed more soldiers and grandfathered those marriages when they phased out that law later - this is how Socrates wound up with his two wives. In ancient Egypt plural marriage was the rule for the royal family and often quite common for the nobility, depending on the time period.

Now, if we look at all those cultures practicing some form of marriage (defined as formalized family bonding, not his 'one man/one woman' revisionist crap) you find not only written records, often tax documents, but also large amounts of artwork. Marriage ceremonies on plates done for the occasion, painted on the walls of tombs. In frescoes and mosaics. They're all over the place as they were a key thing in the culture.

But if we look at more ancient art - those depictions are absent. You see all kinds of things in cave paintings, but nothing that looks like a marriage ceremony. At least not that I've ever heard of or seen. Given how prevalent these are in later artworks across many cultures, it seems a pretty good indicator that, at that time, there simply was no form of marriage that had any kind of cultural significance and quite probably nothing of the sort at all.