I'm writing a science fiction novel set 100 million years in the future, but humanity is still the same basic form we have today, biologically speaking. Would 100 million years of evolution drastically change our biology or would scaling back the time frame to 50,000 years in the future be a better plan?
There is no way of telling what humans would have become after 100,000,000 years. None at all. Evolution doesn't have a plan or a set course. Natural selection leads all species towards being better-adapted to their environment, but in that vast period, there's no telling how the real world environment will change. As an author, you can decide that for your story, but the ways in which humanity would change in response to any specific environmental change are not obvious.
The additional twist is that in the past few thousand years we've begun to modify our environment significantly, and now live in large numbers in many places that can't naturally support them (e.g., the water supply in California, or artificial heating in high northern latitudes). We don't really know how we'll change now that we can set aside many normal environmental factors, or if we'll be able to carry on doing so for thousands of years, let alone millions.
The classic SF novel Last and First Men addresses these issues: over its span of two billion years, many human species rise and fall. It's well worth reading if you want to write on this scale.
The 50,000 span is easier and safer, if less ambitious. Humanity will probably still be recognisably the same species over that span.
Let me put it this way; primates have been around for only 50 million years (give or take). Of this 50 million years, humans have been around for only 2 and recorded history has only been around for about 14000 years. It is extremely unlikely that human would look even remotely similar 100000000 years from now. Even 50000 is unlikely for human to be the same.
Though there is a spark of hope; for the first time in human history humans are selecting mates based on love and not biological features, so it is possible for either of these goals to exists, however unlikely.
Organisms can exist largely unchanged for tens of millions of years if there is no real reason they should evolve. Evolution is a two-way street. So while humans have evolved very quickly up until now, there are plausible reasons why humanity could not further involve. I would suggest looking at Sharks and Crocodiles as examples. Both of these species have existed largely unchanged for tens of millions of years with primary differences being in size and other aesthetics. Humans will definitely change aesthetically, most likely larger eyes and less hair, but in the large part you could force them to be pretty much the same if they are so perfectly suited to their environment like a Crocodile is. You could even introduce this artificially through technology.
I would be shocked if homo sapiens exists 100My from now.
Either we will have done ourselves in by then or we will have advanced far beyond what we are now. There are a bunch of very stupid things in our genes, I would be very surprised if we haven't cleaned up our genome by then even if we have done nothing else. Fixing the flaws will make an organism that can't breed with Mark 1 humanity and thus it will be a new species, not Homo Sapiens.
In practice I think we would be many species by then as some groups decided to make changes that others didn't want to do. That's assuming we are even biological entities by then.
If there are no technological interventions, then humans will have long evolved away from their current form in just an eyeblink of time. Our remote hominid ancestors fist emerged about 5 million years ago, and distinctly hominid species can be traced back to @ 2 million years ago. Depending on how the environment changes, humans could evolve into almost anything (imagine a seal like creature evolved to hunt for food in the water).
There were two books by Dougal Dixon which looked at this idea way back in the 1980's, the first was called "After Man", which supposed that the human race became extinct along with a lot of other species; creatures like rats evolved to fill multiple niches (deer like creatures to sabre toothed predators) as one example.
"Man after Man" did the opposite, technology and civilization failed and human beings evolved to fill various niches, including some who became herbivores and others which evolved to prey on them (due to the damaged environment and genetic engineering, evolutionary pressure selected against general intelligence in favour of instinctive behaviours to ensure survival; an author's handwave).
If any human creatures existed 100 MY from now, they would almost certainly be recreations done as a hobby or art project by very advanced post humans, AI, or aliens (see A.A. Attanasio, "The Last Legends of Earth", set a billion years in the future).
Is your novel set on Earth or on other planets too? If there are people on other planets, natural selection will be altering them to adapt to, say the low gravity of Tau Ceti IV or the high uv of Sirius B III.
If they are stuck on Earth all that time, then there will be a few 'dinosaur killer' asteroids hitting, and mega volcanic stuff as continents drift around, along the size of the Yellowstone supervolcano blowing its top. Might be able to pack in a few ice ages too. All of those will potentially (unless your people have super high tech to prevent the disasters) cause periodic and massive population crashes.
Whenever there is a population crash, there is potential for genetic diversity to be lost by accident. And for previously rare genes to become common by accident.
Imagine, for instance, an asteroid hits tomorrow, and people only survive in large numbers in Australia. Future humanity would only have the genes and physical characteristics of the ethnic groups currently in Australia. Plus whatever new mutations arise and whatever they genetically engineer into themselves.
You might want to reign in your timescale. We're already tinkering with genetic engineering and mechanical enhancements 10,000 years after we first scratched runes into rock. "Humanity" could become a machine race in just a few hundred years if the transhumanists have their way.
Imagine explaining the concept of a machine-borne infomorph - or even the internet - to a caveman. Now imagine how incomprehensible things will be to us when given 10,000 times the advancement. In 100My our descendants would be as far past us as we are past a protein chain.
There's a rather haunting story (Baxter?) which I read, following humanity pretty much unchanged from the survivors of the disaster which ends our civilisation, to the approaching death of the planet as the sun starts to expand. Actually, more a set of word-pictures scattered across a billion years, rather than a story with a plot.
The underlying thesis, is that humans cannot regain a high-tech society because we have dug up and dissipated the key resources accessible to low tech. Future attempts, after millions of years of geological "recharging", will fail to reach our level due to scarcities, but only after again dissipating the key resources.
On the other hand, evolution will do little to re-shape us, because humanity even in a primitive state, changes the environment to meet our needs, rather than being changed by the environment. We kill the predators. We protect weaker members of our tribes. We uproot the weeds. We plant our crops. We tend our flocks and herds.
There are holes in the arguments. For example, evolution can be driven in unexpected ways by sexual preference. That's how the Peacock's tail and the Bower bird's bower happened: random accidents of the female bird's preferences spread over millions of years. Likewise, I think that a high-technology civilisation would eventually be re-attained by humanity, if we don't go extinct after the fall of this one. But equally, there has never before been a species that modifies its environment in the way that (even primitive) people do. So just maybe, recognisable humans will be around until Earth dies.
Food for thought:
Evolution via natural selection has ended
Survival of the fittest no longer applies. In fact, humanity has become extra conscious not to discriminate against any physical or mental disadvantage, but to make an effort to include and support them. This means any of those traits which may be genetic have a much better chance of remaining than in past scenarios.
Evolution continues externally through technology
In the past, if it got cold, the warmer/hairer beings survived and flourished. Now? Just put on a jacket. Not as fast as your fellow humans? Get a bike, or a car. Instead of forcing biological change, any difficulty in our environment now brings out technological improvements.
So, now our "evolution" is voluntary and conscious, the question becomes: How do we want to change? And, when do we want to change?
Natural selection is the primary motivating force behind evolution. It is the mechanism by which a group of organisms is more likely to pass on traits beneficial to their survival as the individual members who pass them on are more likely to survive. For example, giraffes with longer necks are more likely to find food then their short-necked friends, and are thus more likely to survive and pass their genes on to the next generation, resulting in longer-necked giraffes.
As all humans, without regard to their genetic traits, survive to pass on their genes to the next generation, and there are no traits which make you more likely to survive (and eventually procreate), humans do not evolve by natural selection.
That said, recent research does show minor changes in humans over time such as smaller teeth, smaller brains and so on. Personally, I think these changes do not reflect much considering that technology changed our lives so much faster than evolution ever did. That means that research conducted, even with humans from hundreds of years in the past, cannot come to any real conclusions considering the technological changes made in this interval (such as nutritional changes, medical improvements, changed lifestyles, awareness to pollution problems and so on).
I also think that as our technology improves we become more and more adept at adapting our environments to us (as opposed to natural evolution, which adapts the organism to the environment), and so lose the need to evolve, and thus, the mechanisms which drive it.
Bottom line, I think that humans in 100 million years will be vastly superior to us mentally, but will probably look very similar to us.
If the time span is basically arbitrary and not relevant to the plot, then 50,000 years is much more believable, and still seems an incredibly long amount of time. Because I think the chances of humanity being biologically the same and still living on Earth after 100,000,000 years are very very slim. So slim that you would need a convincing explanation as to why humans have not changed.
So really, I think the future is so open that you can predict whatever you want. Whether the reader finds it plausible is down to how good your plot and writing and backstory are.
The best way to understand how different the world might be in 100 million years' time is to consider how different it was the same length of time in the past.
100 million years ago, some of the first flowering plants were evolving, alongside the first bees. Some early mammal-like creatures did exist at this point, but without most of the features we would consider to make them true mammals. There were no snakes or ants. This is the age of the dinosaurs, but at this point even many of the most well-known dinos (T-Rex, Tricerotops, et al) have not yet appeared.
Evolution is a slow process, but at this scale the world is a completely different place. Entire classes of species have risen and fallen multiple times in that space of time. You should read the evolutionary timeline article on Wikipedia, as it can give some good insights into the topic.
Others have commented that "evolution has stopped for humans". They are basing this assertion on changes in our behaviour that have occurred in the last dozen or so generations or less. This is completely the wrong scale to think of evolution. Evolution hasn't stopped; it just isn't noticeable at that scale.
In the absence of world-changing cataclysms, you'd need to be looking at tens or hundreds of thousands of years to see noticeable evolutionary changes.
In a 100,000,000 years I think the safest answer is : We'll be evolved beyond a level we can comprehend. If such an "entity" would be sent back to 2016, it would be seen as a divinity.
Think about humans, 10,000,000 years ago we were tiny stupid monkeys living in trees, 100,000,000 years ago, we were some sort of hamster. Now from a hamster's point of view, we are gods, even beyond that. We can fly to space which is something a hamster's little brain can't comprehend.
In a few centuries at most we will probably have completely mastered genetic engineering. We will be able to "control" our evolution.
Then what we do from there will depend on our goals. Making ourselves healthier would be the first thing we ll do. By that I mean make our bodies naturally more resistant, increase our lifespan, have limbs that can regenerate etc...
We ll also try to make ourselves "better". In such an advanced world, physical strength would be even less relevant than in our current world (where it's mostly for esthetics). I think we ll try to make ourselves smarter.
Then there wil be some evolutions "just because we can". For example water breathing abilities, flyting abilities, all sort of skills from the animal world that can be applied to us.
Then maybe milleniums after that when we reach a level of intelligence orders of magnitude above our current levels our goals would probably be different.
Looking at the hamster vs us, look at 100,000,000 years of slow random evolution achived. Imagine what can happen in the same timespan if it s controlled by some super intelligent being... That s why I think that even the most imaginative sci-fi writers can't imagine how we ll decide to evolve and how we ll be in 100,000,000 years.