The first human stepped foot on the Moon in 1969.
The first probe (Venera 3) landed on Venus in 1970
The first probe (Mars 3) landed on Mars in 1971.
If Venus and Mars were habitable (they have liquid water, and are perfectly safe for "naked" humans except for the absence of food), the resources and effort to put more probes and humans on them would have surely been far greater.
By what year would they have been colonized?
1990.
The moon race was really the peak of the "space race" between America the Soviet Union. The Soviets putting something in orbit was A Big Deal, and America needed to not only answer that, but show they can do way more. Once on the moon, though, there simply wasn't much interest in continuing. The space race was basically over.
But imagine if we had been looking at Venus and Mars, and they looked like they could be habitable, so we sent probes, and found out they were habitable. The space race would not have ended. Getting people (and military...mainly military, really) to lay claim to these planets would be absolute tip-top priority. Mustn't let the Soviets get there first! And the Soviets would do their best to try, because they mustn't let the Americans get there first! I think -- much like the moon landings -- the practicality or immediate value of doing such a thing would take a deep back seat to the idea that it must be done.
I think you'd see America care a lot less about the spread of terrestrial Communism. It wouldn't have to fight in Korea or Vietnam. They'd be busy getting some rockets into space, and pouring loads of resources into that. Who needs this little bit of land in Southeast Asia when you can grab entire continents on Venus and Mars.
There actually was a plan, in 1969, to get people on Mars by the 80s, including a 50-man earth-orbiting space station, but it was far more costly than the moon landings and the plan was not well received. But I think that would be a vastly different story if we knew they were fully habitable.
tl;dr The space race would not only have not ended, but would have become a bigger budget item than anything else, and top priority for any country or alliance that could cobble together anything to send there, and in 1969 we felt like mid-1980s was a reasonable, if ambitious, target, to start landing people there.
By what year would they have been colonized?
The answer to this question is heavily dependent on what resources we are able to find there. In this instance, Venus and Mars would obviously have the great benefit of an additional place that was safe for humans to live.
You'd see the timeline start diverging mid century, where we're starting to get better observations of Venus and Mars. I'm going to make the assumption that you don't want drastic differences to the two planets - Venus still being obscured in mysterious clouds, and Mars still being primarily red.
Going into the 1950s and 60s, we would have some awareness that there was liquid water on Mars - if there are large seas that is, we would probably be able to detect them via telescope.
The point of real divergence happens when these respective probes land on the planets - Venera finds a lush rainforest. Viking finds a desert, but nonetheless, one that shows signs of life.
From here the prediction begins to get fuzzy. In our real history, while significant fractions of the public protested the Moon landings as a distraction, many in the public were confident that we would soon be going to Mars, and were excited about this "inevitable" sci fi future.
The problem is, getting to space is still pretty difficult. The tyranny of the rocket equation makes getting things and people into space expensive, and thus, in the 1970s, with the Vietnam war, an energy crisis, and unrest as we tried to figure out the aftermath of the civil rights movement, congress was very uninterested in exploring space. There is just as much of a likelihood that NASA could've gotten completely dissolved or just greatly diminished rather than what we got.
What got NASA to keep moving was the promise of the space shuttle, a much cheaper, reliable craft to Earth orbit. After establishing this, we would build space stations, and cheaper architecture to return to the Moon, and truly establish an in-space economy.
This however, was not to be. The shuttle was vastly more complex than NASA and its contractors had intended, and after the Challenger and Columbia disasters, it was solidified as having failed in its purpose. It did many great things, but had failed in its goals nonetheless.
Flip flopping priorities from presidential administrations, and lack of interest from congress led to little progress in space until the 2000s. By this time, NASA couldn't really be killed off, because they were providing so many jobs around the country, but neither could they really progress, as a plethora of factors, mostly political, were holding them back.
The thing that really changed the tide were the commercial companies. SpaceX, Rocket Lab, and many others coming onto the stage in the past twenty years really disrupted the industry and have lowered the cost. The true consequences of this I feel we'll only really see by the end of this decade.
By decade's end, we will likely see at least one commercial space station, Axiom being the most likely, but we could have as many as three or four (starlab, orbital reef, Northrup Grumman station).
Meanwhile, NASA is finally getting the political support necessary to return to the Moon, and SpaceX is gunning for Mars. I am personally more optimistic than some in my predictions - and thus I say that we will probably get people on Mars sometime after 2032, and I think it's very likely we will get there before 2050.
The cost of spaceflight is decreasing dramatically, and with commercial space stations, the demand for human spaceflight will also increase, leading to more reliable and cheaper flights for people. Thus, while many engineering challenges lay in getting to Mars, it is for the most part a money problem. We could've tackled the engineering decades ago, I would say. But that money is important - and focus. The commercial companies have shown to be very adept in that regard. Unlike NASA's contractors, they cannot waste money for little development, or else they will go out of business.
Now. Tangent on bringing up to speed the current state of today's spaceflight world, let's return to the original question - how would this be different in this new scenario?
My unfortunate answer is probably not tremendously. The planets being discovered to be habitable would have an immediate effect of excitement in the public - life has been discovered on another planet. You'd probably see congress greenlight more funding for NASA to conduct science missions, especially sending their own missions to Venus.
Actually getting there would be difficult. You'd still have the question many had then, and still today - why explore space when we have enough problems on Earth? This is an overblown feeling by the public being quite frank, considering that the average American pays about $30 a year to NASA in exchange for tons of jobs, scientific discoveries, and NASA developed technologies being spun off into ones useful for consumers.
It honestly is quite up in the air. It all depends on if NASA could get the support behind it to get there faster. They might get trapped in a paralysis of sending many scientific probes, that though making many discoveries, would not be pushing things forward.
Perhaps the commercial sector may spark into existence a bit faster. Whether it would have been as successful is a big question. SpaceX changed the whole paradigm because they wanted to do things quick and dirty. They wanted to break the monopoly that Boeing and Lockheed Martin had on space. Whether a similar company could have arisen a decade or more earlier is up in the air.
What I am quite confident in is that SpaceX's iconic reusable boosters tech likely couldn't have come around too much earlier, as it requires pretty good computers controlling the descent. This would have a pretty big impact on the cost of space. It does not preclude other reusable tech coming about, like gliding boosters down to a runaway.
Whatever the case, for your colonies to come about, you either need a world willing to drop buttloads of money, burning hundreds of millions to billions per launch, or find some way to make a rapid reusable rocket - and one that doesn't require extensive refurbishment like the space shuttle.
The problem is that even if you find something valuable, like rare-earth materials on Mars, getting them back is so prohibitively expensive, it wouldn't be like how gold and spices pushed forward exploration five hundred years ago. It could sustain a colony after a great deal of time, after launch costs get lower, but as for pushing the first exploration closer, probably wouldn't do a lot.
One possible idea is that the population scare, most prominent in the 70s (before improvements in agriculture showed we'd be able to feed a larger population) could perhaps prompt a movement to send people to Venus or Mars. This could bleed into political support for space, with congress giving some token support towards larger NASA budgets.
Conclusion then for when we might see the first boots on Mars and Venus? As I said earlier, probably not too much different to our reality. These planets would be a lot more enticing, but many of the challenges are still there. If there were bigger changes, I could see us getting to Mars in the 2020s or even perhaps earlier, but it'd likely be rather ramshackle. How this pans out depends heavily on what decade it happens in, what the geopolitics are like, etc. You could come up with reasonable explanation for it happening in the 90s maybe, but not too much earlier I'd say. You might look at the alternate history show For All Mankind. Not super accurate, but gives enough explanation for a work of fiction.
Venus would be more difficult, as presumably you want to return home on the early missions, and due to the larger gravity, would require a larger return rocket (which is already a difficult task with Mars exploration). I'm confident in saying then that we would probably see Venus happen later, but not extremely later. It depends on if there is competition between countries or companies who might want to take the "real first prize."
And by what year (if different) would those colonies become self-sufficient?
Oh boy. Self sufficiency. That is a tough question to answer. I'd venture a guess that it would be faster than with our real Mars and Venus. It would be slow going I'd say early on. Much more native flora and fauna would prompt a lot more scientific research before colonization could happen in earnest. It would likely lead to protests from the growing environmental movement, that "we're already screwing up our Earth do we need to screw up two more?"
This is already a thing that a lot of people are saying in our world, and we don't even know if there's anything living out in our solar system. It would be much more pronounced in this world. There is also a lot of risks to consider with space originating diseases, astronauts quarantining after missions, and likely a good while before we'd start seeing people walking around on Venus or Mars without a space suit. It would be at least easier that they would only need something more similar to a hazmat suit, and not a full blown pressurized space suit.
If the environments of these planets proved suitable to human life, if the protesting was managed in some way, then you'd likely start seeing colonization efforts arising. It would depend heavily on if you could grow food in the planets' native soil. Mars in real life has very toxic soil that would need to be heavily processed before use.
This depends on if the life on these planets is carbon based, how similar it is to Earth, etc. If its carbon based, and there are ecosystems on the planets allowing breakdown of soil, we could perhaps grow food there - but that would likely only happen after years of astrobiologists studying this new life.
Your early colonists would, for the most part be scientists. Commercial companies would probably start moving in next. There isn't much of value business wise on these planets still, but Mars would be a good stepping stone to the asteroid belt, where mining could be done.
So I'd venture that Mars would not only be visited first, it would also grow faster. Venus would be a more desirable place to live probably, depending on how hot it is, but it wouldn't have much value beyond the science, unless we discover something valuable there.
So yeah... this scenario gets into a ton of "what ifs." It allows you a lot of flexibility in coming up with reasons why something can happen earlier and still be plausible - perhaps "the spice must flow" on Mars. I'd probably make an upper limit that self sufficiency on both worlds would probably happen prior to 2100 (which I think could probably happen on Mars in our world) but technology could make that go faster, environmental concerns slow it down, or politics just not get its rear in gear.
I can for sure say that this change to these two planets would really muddy up things. People will likely be more persuaded by the idea of becoming a multiplanetary species (a goal often spoken of by SpaceX) - obviously people get the idea of having insurance in case of the absolute worst of Earth facing extinction - but knowing that Venus and Mars are actually livable places would increase this allure a lot more. Most people interested in living on Mars today are... eclectic, lets say. It would be a difficult life, where we would need to spend centuries at least in protected bases or underground before we could have a world where we could breathe. This world though, would be much different.
The environmental concerns though, are again a big factor in how this pans out. Today a lot of political discourse goes around about the impact Europeans had on the natives of the Americas - in the future it might be that future Martians and Venusians looking down upon their ancestors for tramping down the native creatures and plants.
This has been a very, long response, but it is a scenario that prompts a lot of questions. You could take it in a number of ways, have many interesting stories to tell. You could investigate how we deal with being the stewards of another planet when we barely know how to manage our own. You could look at how this changes the culture and ambitions of people a thousand years from now. Maybe we care little about pursuing interstellar travel when we have three whole habitable worlds to contain humanity.
I wish you luck as you study this. For further study, I recommend looking at stuff on space exploration (like from the youtube channels Scott Manley and Everyday Astronaut) and stuff on Astrobiology, like the work of Seth Shostak.
Of course, humanity did not really need to take a 50-year break on manned space travel after 1971. We could be on Mars by now. The long pause happened in our world, because the main accomplishment, that was putting a human being on the moon, had been achieved. But private enterprise and governments saw little use for manned moon missions, the moon being a desolate and hostile environment place, without relevant resources.
Limits to Growth: serious incentive to proceed colonizing another planet
Now imagine bringing humans to Mars or Venus would be the new goal set. Why ? There was every reason to consider colonization: in this period (1972), the report Limits to Growth published by the Club of Rome: a limit on the further expansion of humanity on Earth and planetary resource use. There was a serious incentive.
Timeline 1972-2022, let's pick Mars
We reached the moon, but now, technology would be needed to colonize a near planet. A huge project. Governments world wide started to cooperate and invest in it. There were two options: Mars had been known for its deserts, but oxygen (16%) and plenty of water were detected, so there was every reason to assume Mars would be inhabitable. Venus could be, but it seemed somewhat uncomfortable to permanently live indoors.. and most water exists in a boiling state, requiring cooling equipment, that would be expensive. So.. Mars was picked.
Moon base, Mars expedition
The NASA, together with rising private space enterprise (yields were projected!) set out to build a moon base, which was finished in 1985. By 1992, the first astronauts set foot on Mars. They stayed there for a few weeks, with no need to wear a helmet. But they had to take food supplies, and when these supplies were exhausted, they were forced to return home.
Cargo ship
The current absence of food on mars implied agriculture had to be developed.
Meanwhile, ESA and the Chinese enterprise Spactec had developed a cargo ship. By 1988, they finished the design, and a succesful test launch followed in 1994. The Spactec freighter, a one-way, fully robotic space ship, could transport and parachute fuel and tons of goods, which was needed to boot agriculture development on Mars..
In 1996, a Spactec ship was launched empty, and parked on the moon. Then fourty-eight Earth-Moon shuttle Missions followed it, each loaded with agriculture equipment, like tractors, plowers, seeds, soil, embryos of cattle and poultry, and tons of food.
Food production
In 2002, the Spactec freighter was fully loaded and launched from the moon, on its way to Mars. In 2004, a team of farm supervisors and technicians arrived on Mars, with the farming equipment ready for use. With the help of farm robots, they had food production up and running on Mars in 2018.
People
Volunteer colonizers came in plenty: there was a long list of candidate colonizers. Anyone who would like to leave the planet subscribed to a website, submitted a short autobiography and experts in sociology, economy and behavioral psychology evaluated the candidates.
One Spactec cargo ship named Pioneer III was modified to carry people. The ship left for the moon in 2018, arriving on Mars in 2021. The ship had 1600 colonizers on board. At this point, careful reconnaissance followed, the colonizers consuming their first Mars vegetables and meat. They celebrated Christmas on their new planet, with the first home grown Christmas meal in december 2022 and the original (and now famous!) Mars Beer.
Depends on what you count as "being colonized".
A colony, in the traditional sense, has a few specific purposes:
Since space travel is incredibly resource intensive, you won't be able to move significant amounts of people between Earth and other planets, and this might even be a hard, physical limit.
The same thing also limits the transport of resources. There is no point in extracting resources on Mars, if getting the resources back to Earth takes more resources than what fits on the transporter.
This leaves prestige, which is worth something in the right political circumstances (e.g. the race to the moon, which wasn't actually about getting to the moon, but about proving that the USA/USSR is better than the other), but is probably not enough to warrant too much cost (as seen in the real world, where manned space travel to other planetoids quickly stopped once the prestige goal was reached).
In the case of colonizing other planets, there is one additional reason:
That's a mostly moot point. The dead on Earth will not care that there are a few people on Mars still living.
Also, getting a few people to Mars and creating a self-sustaining advanced Civilization on a different planet, that can survive when no more supply ships from Earth arrive, that's something completely different.
A settlement of 1000 people can easily be wiped out by a single disaster.
So I fear, there wouldn't be much practical and financial motivation to actually create a colony on Mars.
I'm sure we'd have a few people on Mars by now, but the Mars colony would be probably on the level of the ISS. A handful people, who are there "for science".
Not too much before present time.
Interest in Mars has always been there, but only in the past 20 years (Spirit and Opportunity were in 2003) have we actually managed to get mobile robots on the ground there. And around 30 missions to Mars failed. It's not as easy as it seems.
If Mars were habitable, interest would've been much higher, of course, and more effort would've been made, but why would technology have developed very differently? More funding and interest can accelerate things by a couple years, but the same way a Moon landing wasn't going to happen in 1945 no matter how much some Nazis would've wanted to escape Earth, colonizing Mars requires technology that we barely have today, even if all you need to bring is people, construction materials and agriculture foundations.
Depending on what was there. Another Earth (minus life)? Nobody would seriously care. We don't even "colonize" the shit out of "primitives" in the various parts of Earth, like in the past. Even if we do, it is because of ideology, not because we want more land. In that case, we would be only trying to send people there to show we can, not because we cared about that piece of ground. So, it would happen slightly faster than now, only because one-way trip makes some sense.
But if people knew there was some valuable unobtainium out there in addition to a place to live, we would be SERIOUSLY trying to get some of that. Bonus points if that unobtainium can be only extracted/handled/transported by humans, probes/robots being too clumsy. In that scenario, it isn't too far-fetched that NASA would be given way more money to work with, which would seriously speed up the timeline of rocket development. Obviously, at a high cost to other areas of science - like how putting everyone on Manhattan project stopped or slowed down everything else.
Now the important question is, when do we learn of this unobtainium? If we got to know with the first probes in 70s, then possible timeline is in 1980s. Under huge and likely unrealistic assumption that USSR would simply watch and let USA get hands on that, and not actively try to sabotage their efforts in various ways (having USA+USSR work together is too far fetched while I believe USSR wouldn't have a chance to win the race). If we learned that back in ~1950, that might bring timeline down to as short as 1970 - it would be a MUCH more worthy goal than merely getting some beeping chunk of metal in orbit.
The formation of the Earth is believed to have occurred about 4.54 billion years ago. Prokaryotic life originated not too long after, as early as 4.28 billion years ago.
In their early histories, Earth and Venus likely had thinner atmospheres than they have today; Mars likely had a thicker atmosphere than today which may have been similar to early Earth's.
These relatively thin early atmospheres, coupled by a preponderance of projectiles striking the Earth during the Late Heavy Bombardment, 4.1 to 3.8 billion years ago, could very well have allowed meteorites seeded with prokaryotic life from Earth to arrive on Venus and Mars.
Over 53,000 Martian meteorites have been found peppered across OTL Earth. Today, Earth's relatively thicker atmosphere would make it less likely for Terran meteorites to land on Mars, but this may not have been the case ~4 billion years ago when the two planets had more similar atmospheric densities. Likewise, Venus's atmosphere today is almost 100x as massive as Earth's, making it extremely difficult for Venusian meteorites to escape - but, again, this may not have been true much closer to both planets' origin date. It's not unreasonable to imagine material being exchanged between all three planets in this era.
If, unlike in OTL, environmental conditions on Venus and Mars had gone down a path more similar to that of Earth, then microbes on Terran meteorites might have been able to reproduce, flourish, and evolve on those planets. Over the eons of evolutionary time, entirely alien alternate trees of life could have evolved on Venus and Mars, eventually spreading into every ecological niche on those planets.
Well... perhaps not entirely alien. For one thing, these separate Venusian and Martian trees of life would still have shared a common evolutionary ancestor with Earth life. Thus, we might expect it to also have some of the same properties as the Last Universal Common Ancestor (LUCA) of life on Earth: its biology would be based on carbon and water, its genetic material would be DNA or RNA, its metabolism would use the same amino acids, it would share other aspects with Earth life such as enzymes, proteins, ribosomes, etc.
And for another thing: "habitable" is, at the end of the day, relative. If by "habitable" we are assuming that the Venusian and Martian environments went down similar paths as Earth's environment, then via convergent evolution we might expect some traits to originate independently on Venus and Mars in parallel with traits of Earth life. For example, the abundance of energy arriving at all three planets from the Sun may have spurred the independent evolution of photosynthetic life - though most likely, the specific chemical pathways would differ drastically.
Nevertheless, these commonalities put only very weak constraints on just how alien life on Venus and Mars could have gotten. At the very least, we might expect to see at least as much diversity across planets, as already exists within the single tree of life which evolved in OTL on Earth - and the amount of diversity we already see in the latter is staggering. Life on Venus and Mars would likely have evolved into categories which never existed in OTL - though, whether these categories would be practically relevant from a human perspective is uncertain.
As for the prospect of sentient life: here I delve a bit more into personal subjective speculation. My take on it is that we tend to be guilty of highly anthropocentric thinking when it comes to speculating about nonhuman "sentient life". We see Earth life has having evolved according to a teleological progression, i.e. from the simplest single-celled life, through less to more complex organisms, and finally arriving at us, "sentient life". We see this history as a "progression", because it already happened so here on Earth; but from a pure evolutionary perspective, this is nonsense. Evolution has no "end goal"; it just happens. Thus there is really no good reason to imagine that Venusian or Martian life would happen to converge towards sentience. "Sentience" is, imho, a circularly-defined notion: humans are sentient, and sentient means "like humans". Venusian life could just as well end up "progressing" toward some other characteristic which, from our perspective, seems utterly arbitrary: sphericalness, bumpiness, symmetry, viscosity, anything at all. Martian life, as it increased in complexity, could end up "progressing" toward some other property, which while roughly analogous to our own sentience in a very loose sense, is something of which we cannot even conceive and for which we have no concepts.
Human colonization of Venus and Mars in this timeline would likely be a rocky prospect. In the early days, most likely humans would need to live in sealed colony habitats ironically reminiscent of those in OTL, though the reason for such habitats would be different: there would be no telling how alien organisms which evolved on Venus and Mars might interact with our own Terran biology. For this reason, the early cost/benefit of extraterrestrial colonization would not be too much lower than in OTL; thus, the timescales on which it occurred might also end up being similar.
Eventually, we would ascertain whether/if/where it was possible for us to interact with the native Venusian and Martian biospheres without danger to ourselves. This would likely be a pretty gradual process: after all, we still have only an incomplete picture of biology on Earth, and we would need to understand entirely new xenobiologies from the ground up. Considering our psychology, it would likely also be a process fraught with various tragic accidents and risky experiments - I imagine not a few people would end up just saying "screw it", and jumping out of the habitat, damn the consequences. The colonization process writ large would be replete with ethical conundrums and existential risks to humanity needing to be managed.
If we subjectively judged Venusian or Martian life to be inconsequential and/or dangerous, we might simply try to sterilize it and substitute it with more familiar Earth organisms. Alternatively, research access to entirely new xenobiologies might spur medical, ecological, or other lines of research in a way that is simply not possible in OTL.
Last, but not least: access to Venus and Mars would reshape and widen our conceptual understanding of exactly what "life" is, and of the myriad ways in which it can vary. While we would still be constrained by "solar-system-centric" thinking due to lack of referents apart from the genealogies of life originating among the inner planets, we would have access to larger & entirely new categories of knowledge which are not available to us in OTL.
