There are basically two kinds of kisses: the social kiss between friends and family that is just a pressing of lips against skin, and the open-mouth sexual kissing between lovers. This research involves the latter.
The oldest kisses documented in text has been pushed back to 4,500 years ago, found in Mesopotamian cuneiform writing. That's a thousand years earlier than previously known documentation. But human writing only goes back so far, and since kissing has been found in such ancient documents, it may well have been an established custom before writing existed at all. Evidence of kissing before that relies heavily on how we interpret the clues.
We know that humans interbred with Neanderthals, but did they kiss? A DNA study of Neanderthal tooth plaque found a 48,000-year-old microorganism that was rare in Neanderthals but common in human remains. While that could be evidence of kissing, there are other ways it could have leapt from one species to the other.
However, bonobos, one of our closest primate relatives, kiss as a precursor to sexual behavior, while chimpanzees only employ social kisses. That may lead us to believe both kinds of kisses were already present when homo sapiens emerged. But it's not conclusive evidence.
These questions come from an article about a study that looked for a correlation between ancient text references to kissing and the spread of disease. You can read about that study, and the history of kissing, at Smithsonian.
(Image credit: Francisco Osorio)
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