TLDR: Real estate
https://www.ft.com/content/99a3cf9b-0ab8-45b9-bbc5-7e88c08f9ea5
Paywall.
I don't think it's the oldest asset class though. That would be gold/jewels/shells or livestock. Nomads could amass very small amounts of wealth and probably did.
New analysis shows that the switch to agriculture did not lead to massive wealth inequality. That took another several thousand years. It was the use of oxen that enabled inequality (that's based on northern European archeology... I suppose it could be different elsewhere). Essentially, before oxen there was no way to convert land into wealth, because a given human could only work about as much land as another, with minor difference for ability and motivation. When people learned to harness oxen, wealth was limited not by the amount of land a family could till, but by the amount of land a family could own, because with wealth you just buy more oxen and start to hire labor.
Anyway, I know that's all besides the point. I've seen other things that say that most of the wealth gap between white and black in America can be explained by the differential in home ownership, which is in large part explained by the difference in access to federal loan programs (GI Bill, Fannie Mae, etc). Before those programs, the average difference in black and white wealth was supposedly smaller.