Please remember that asking more than one question is quite literally a reason to close a question. Click "close" and read the definition for "Needs More Focus." When you boil it all down, you're asking two distinct questions: would there be any evidence and at what point could that evidence be positively evaluated.

Also, please note that you have what I'm fond of calling a "technology dichotomy." Your alien humans had the technology to travel from their planet to Earth, but once here, they're stuck with 2024-level tech? Why? They're either traveling via a generational ship or FTL (if it can be done). That's some serious Clarkean juju there. Aaaaand finally. 5 million sounds like a lot, but I doubt it's enough to support 2024 tech (much less later). You need a LOT of people to support high technology because some of your population are children, aged, employed in support roles, etc.

(1) It is not possible to have "technology at least on par with 2024, with a population of about 5 million". At least 500 million people are needed. With only 5 million people they could maybe have technology on par with 1024 AD. Maybe. (2) We know for sure that there were humans on Earth for very much more than 100,000 years. We have known that since, I don't know, the 1960s? (3) We know for sure that there were no modern-style cities with millions of inhabitants in central Africa 100,000 years ago. Cities are big things, which are hard to hide; e.g. think of all the copper power lines.

In fairness, the poster didn't claim that his 5 million people at 2024 tech was sustainable. The whole point of the question is that they died out.

Reminds me of "Probe 7, Over and Out", Twilight Zone 5x09.

@Jay: It muat hve been sustainable enough to build the cities. Tens of millions of tonnes of steel and concrete, millions of tonnes of copper and glass, millions of kilometers of wire, hundreds of thousands of kilometers of piping, water treatment plants, power plants, hundreds of thousands if not millions of vehicles and so on. They had to maintain their technology well enough.

@RonJohn Reminds me of Xenu "the extraterrestrial ruler of a 'Galactic Confederacy' who brought billions of his people to Earth (then known as 'Teegeeack') in DC-8-like spacecraft 75 million years ago, stacked them around volcanoes, and killed them with hydrogen bombs";-)

I challenge the idea that you couldn't maintain a higher level of technology with a small (5 million) number of people. We can't currently do it because we must store know-how in human brains, pass it on through inefficient human learning processes, and maintain a large enough labor force to extract natural resources. A couple of centuries earlier would have lower requirements, and a couple of centuries further would see advances in automation. We're just at a bad point right now.

Here is an idea. You said "cities in central Africa". In the past most cites were built in areas with good transport, either on rivers (Babylon, Rome, London) or near harbours (Athens, New York, Copenhagen). If your aliens have some other form of transport (flying saucers, teleporters), they might choose the sites for their cities using other criteria,so they aren't in the sorts of places where archaeologists normally look. (This doesn't help with any of the other problems, however). Hmm, what if your aliens weren't in Africa? Proto-neanderthals? Proto-Denisovans, from the planet Dennis?

@RonJohn also Douglas Adams

You might play with having ET's colony ship be a shoebox full of Sufficiently Advanced Technology souls, instead of a bunch of biological organisms. This also gives them a reason to land in Central Africa (Homo Sapiens are there to bodysnatch), even though it's probably the worst vaguely temperate region on the planet to try to start a technological civilization on.

Besides historical evidence, there is also the biological (or evolutionary). Unless the settlers brought all animals with them, then no, it is not possible. We can genetically trace to our ancestors to millions of years ago and we can also find common ancestors with most of the modern day animals. Something like this: research.amnh.org/paleontology/perissodactyl/f/mammal_tree2.‌​png

If you enjoy this kind of thing, see "Inherit the Stars", By James P. Hogan.

@alexp "Millions of tonnes of steel and concrete ..." That's a different issue. I was replying to the claim that one could not sustain a 2024 level of technology with only 5 million people. My point was most definitely not to say that every argument made in this thread is invalid.

@ACV "Unless the settlers brought all animals with them..." Interesting. I wonder whether it would be possible for an alien to live on terrestrial plants and animals, since the amino acids and sugars are likely to be different? [I know Peter Jackson had ](youtube.com/watch?v=7IHwKJOZZ6U) some thoughts on the subject.

@RobertRapplean Even if you could get around the educational problem (and it would be a showstopper for 2024 tech) you have the problem that there will be nobody to fill any position that you need less than one in a couple million of.

@SimonCrase Or how about David Weber's Dahak series. Humanity didn't evolve on Earth but the ecosystems match because Earth life is mostly what was reseeded by the aliens (homo sapiens) after past extinction events.

@LorenPechtel So your aliens outdid Noah bigtime? Not just people, elephants, unaccountable species of beetles, E. coli, Covid (and bats), dinosaurs, trilobites, Lokiarchaeota,...

@SimonCrase "My" aliens? To give a scale on which they operated--Earth doesn't have a moon anymore. The object in our sky is one of their ships, hidden beneath a thick later of the original moon.

That the worldwide religion was praying to porcelain goddesses would be obvious for up to 1M years.