Were there humans 1.5 million years ago?
Anthropologists and biologists don't draw the line between "humans" and "ancestors of humans" at the same point in time. (And from what I can tell the members of either discipline may not even have a good consensus among themselves. After all, it's just words.) But there's not much disagreement over the most conservative placement of that line, which is 2MYA when genus
Homo split off from an earlier genus, possibly
Australopithecus but there are other candidates. (You're the career biologist here so you should be able to make more sense out of this material than I can.
) All of the species of
Homo, such as
habilis, erectus and
neanderthalensis, are called "humans" by most writers.
Some scientists use "human" as a convenient description for all species in the line that broke off from the chimpanzees, our closest relatives. I usually see that delineated around 7MYA, but the fossil evidence of the hominoids of that period is scant due to soil conditions. So not only is the date vague but there's even some controversy over the sequence of the whole Great Ape split into gorillas, chimpanzees, orangutans and humans.
How do they date the footprint?
Well geeze girl, now you're asking about the fundamental techniques of Paleontology 101. I don't know the answer but I wouldn't be expected to since my degree is in accounting. Yours is in a science so you would at least be expected to know where to look for the answer if you were interested!
How do they determine the age of any rock? Carbon dating? Painstaking reconstruction of movements within the earth's crust? Obviously this must be a mature scientific technique since they've been dating fossils since the 19th century or maybe even the 18th. If I wanted to know, I'd ask a scientist. Fortunately we have a few among the membership.