Was the average person no better off in 1800 than in 100,000 BC?
In a detailed review of Clark's book, concluding that its "central theses ... are contradicted by well-known evidence" (p 969), Robert Allen addresses the claim about 1800 and 100,000 BC in a section headed "Malthus in the Very Long Run" (pp 951-5). His criticisms of Clark include:


That he has no evidence of living standards in 100,000 BC, but relies on an assumption that living standards at that time were the same as those of modern foragers (p 951).
That he draws inferences about diet (a key aspect of living standards) from male heights (from skeletons) but ignores evidence from female heights and from other skeletal features, which some have claimed to support different conclusions (p 952).
That his argument that humans of 100,000 BC had more leisure time (another aspect of living standards) because of their forager life-style is not supported by skeletal evidence comparing (more recent) foragers and agriculturalists (p 955).


Some other criticisms in this section are hard to summarise briefly. In conclusion of this section, Allen states: "Economic growth before the Industrial Revolution was not rapid, but it did generate a higher standard of living for most people than that enjoyed by ancient foragers".