I believe this, though in no position to prove it. Among reasons to suspect it, there's the complexity of ancient languages like Sanskrit, compared to numerous modern languages; and the physical vigor of our ancestors, versus generations of ease and indolence; and the world wars, plus the 1918 flu that severely struck the young adults.
If you look at the Neanderthals, we were stockier, had thicker bones and bigger brains. Amazing. Look at kids today- they don’t even know what gender they are. So weak. It might be the soy, the chemicals in the water supply, plastic that emulates estrogen- who knows. We are literally running on fumes. We have gone from wolves to poodles. If we actually could clone people properly, we could use DnA from our ancestors. Get DNA from battlefields because the best of us, the strongest have been pissed away in the endless wars. Unfortunately harvesting sncient DNA is beyond us. Clones always turn out weaker than the original. We used to produce Isaac Newtons, now we produce diddys