I'm going to assume that the OP's question means intelligent life that develops some form of a technological civilisation, as humans have done on earth. There are many other lifeforms on earth that arguably exhibit some forms of intelligence, for example adaptive tool use; we can think of some of the other primates, some birds, perhaps the cetaceans, even the social insects, but none of them have developed a technological civilisation of the type that currently exists among humans.
OK. What do we know?
1. age of the planet earth 4.5 billion years
2. bacteria developed about 3.5 billion years ago
3. evolution of vertebrates: about 525 million years ago
4. permian extinction, leading to age of reptiles (dinosaurs): 252 million years ago
5. cretaceous extinction, leading to age of mammals: 65 million years ago
6. earliest hominids: about 5 million years ago in the pliocene, although some people stretch the definition of hominids back into the miocene, but that is really stretching it IMHO
7. homo erectus - about 2 million years ago; the most successful recent hominid in terms of length of survival, but did not develop any technological civilisation
8. homo sapiens neanderthalis - about 0.5 million years ago
9. homo sapient sapiens (us, modern humans) - around 0.16 to 0.09 million years ago (we are an extremely recent evolutionary development)
9. earliest known agricultural civilisations with complex social organisation that built cities eg Sumer about 7500 years ago (~5500 BC at its earliest). It's important to realise that this is extremely recent in terms of geological time.
en.wikipedia.org
10. industrial revolution: commenced about 275 years ago (~1750); as far as we know, disregading any unproven earlier possibilities such as the Silurian hypothesis, this is the first moment in earth's geological history that a modern technological civilisation arose, involving the application of the scientific method, the widespread adoption of powered machines and large scale exploitation of fossil fuels (as coal, initially). This is an even more extremely recent event. Since that date, the level of technological sophistication, resource exploitation, impact on the global biosphere and world population have increased at an exponential rate. We are still living in the industrial revolution today.
en.wikipedia.org
11. world population reaches 1 billion: 1804
12. invention of RF wireless communications: around 120 years ago
13. world population reaches 2 billion: 1927
14. invention of penicillin
15. first atomic bomb detonated 1945
16. building of jodrell bank radio observatory 1945
17. invention of the transistor: 1947
18. first hydrogen bomb detonated 1952
19. invention of the silicon chip: 1959
20. invention of the microprocessor: 1971 - 54 years ago
21. world population reaches 4 billion: 1974
22. start of the world wide web: 1990
23. world population reaches 8 billion: 2022
24. first early demonstrations of technology that might conceivably lead to the development of AGI: 2023 (eg, chatgpt)
25. 2023 study finds earth is currently in another mass extinction event -
https://studyfinds.org/earth-mass-extinction/
26. 2025 elon musk says humans need to settle mars in order to survive
https://www.unilad.com/technology/space/elon-musk-mars-plans-human-survival-616552-20250601
24. 2100(?) possible development of AGI and the singularity, which may in turn result in the destruction of the human species, as warned of by some AI scientists
25. 2100(?) timeframe - catastrophic damage to earth's biosphere resulting from human-induced climate change, toxic chemical and radiological pollution, soil loss, microplastic pollution, collapse of agriculture, sea life collapse, etc, etc, etc. Not including the very real possibility of nuclear war and the ensuing nuclear winter.
So: out of the 4.5 billion year age of the planet earth, the fraction of time that a technological society has existed that is potentially capable of communicating with intelligent life in other star systems is extremely, almost infinitesimally, small; if we take, say, the construction of jodrell bank and similar radiotelescopes as the first time the necessry equipment existed on earth, which is the last 80 years. In that same time period, the technological civilisation on earth has initiated what appears to be another mass extinction event, which may in turn result in its own destruction.
There is no planet B. Long term survival on mars is a pipe-dream in my opinion, we do not have the technology to terraform mars and establish another biosphere there, there is no "genesis device"; sorry Elon, but we have to look after this planet (I'm sure Elon knows). We are unable even to establish agriculture in antarctica on our own planet without relying on supplies from elsewhere on earth. We know that without the earth's natural ecosystem, humans cannot survive for very long. You only need to travel 100 km straight up from any point on the earth's surface to the Karman line, and from there on out there is an infinity of nothing, no life that we know of, and nowhere that life can exist; it's just 31 miles above where you are sitting, right now.
The technology for large-scale destruction of the earth's biosphere already exists, and the increasing human population level and associated competition for resources raises the probability of that occurring. The bulletin of the atomic scientists clock is currently set at 89 seconds to midnight, so it's very close:
https://thebulletin.org/doomsday-clock/ . Technology and medical science prevent the usual corrective processes that occur when massive overpopulation of one species occurs, such as famine or the evolution of a deadly pathogen, so biosphere homeostasis has not so far been restored. It appears that the evolution of an intelligent technological civilisation results, within an extremely short timespan (in geological terms) in a critical event for the host biosphere.
In summary, if we consider the earth's lifespan over geological time as a model, then (a) the probability of the evolution of intelligent life which goes on to develop a technological civilisation is extremely low, and (b) it appears that the development of a technological civilisation on earth is pathological for the host ecosystem and risks the destruction of both the host ecosystem and the technological civilisation itself.
Hence if the same rules apply on exoplanets, then the probability of any two planetary technogical civilisations arising sufficiently contemporaneous to each other to be able to communicate, let alone visit, is vanishingly small. In other words, the 'fi', 'fc' and 'L' terms of the drake equation are very small numbers when considered at any specific moment in time. Hence the fermi paradox. The 80 years since 1945, or the 275 years since the start of the industrial revolution, may seem like a long time when compared to our own lifespans, but on the scale of geological time it's not even a tick of the clock.
We can speculate that the current mass extinction event on earth is an example of a 'gate' through which technological civilisations either pass or fail. Admittedly, the Earth is only a single data point, so it is possible that some technological civilisations in the universe may have got through the host ecosystem collapse 'pinch event' and survived, or indeed not have experienced ecological destruction themselves. That then raises other possibilities such as the 'dark forest' concept, that is, there may be some other people out there, but they're keeping very quiet for fear of being discovered, as Stephen Hawking warned us about.
So, to answer the question. There may well have been thousands of civilisations formed by intelligent life out there over the lifespan of the universe, but that is over a very long period of time. As to how many are out there right now, at the same time as we are here, I suspect the number is rather small, because of the arguments I have presented above. There may be billions of exoplanets in our local galaxy, but there have also been billions of years when nothing but bacteria, plants and non-technological animals existed on earth. And once a technological civilisation does appear, all the indications from our own experience suggests that it may not last for very long. As for other galaxies, the distances from here are simply too huge for the possibility of contact to be any more than pure speculation. If there really are things like Dyson sphere's out there
https://en.wikipedia.org/wiki/Tabby's_Star then they are so much older and far ahead of us technologically that it is impossible to conceive of what they may be like, however, it may be wise to keep quiet, as Hawking recommended. A planet with a viable ecosystem of carbon-based lifeforms may well be a rare and valuable asset; the earth is our only real treasure.
Humans are at a very early stage of their development as an intelligent llifeform that has developed a technological civilisation.
There are four tests (that I can think of right now!) that humans must pass in order to survive in the near term on earth; these are major challenges for humans as a species:-
1. humans must not destroy themselves by compeition amongst themselves for limited resources;
2. humans must not destroy the host ecosystem, which is their life-support system on spaceship Earth, without which they will perish;
3. humans, being a monoculture, must survive the concurrent evolution of deadly pathogens, covid-19 being a recent example;
4. humans must not replace themselves with some form of machine intelligence that leads to their own extinction.
I'm sure this is not an exhaustive list.
Personally I want us to make it through our own ecological pinch event, but it's not looking very promising at the present time. It will be very sad if we don't make it.
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