Besides one short-lived foray into blitz speed, I've used all default settings for multiple starts now. It seems like everyone leaves Neolithic before I can (thus snapping up most of the good cultures), and there's always at least one AI that's hitting Classical before I'm even halfway done getting enough Ancient stars. Considering that the default difficulty is the second-to-easiest, and that a common complaint has been that the AI is too easy even on higher difficulties, I can only assume I'm missing something. Does anyone have tips? For context, I'm a Civ vet but I always play on Prince or below, and I am an almost complete newbie to the Amplitude style.

You have to keep in mind that you can transcend your current culture, that is to say, keep it through the next Era as well. Also staying for longer in your current Era may not be that bad - if you are a Scientific culture you can still research techs from the next Era. You will also be able to get more Fame score getting all the other Era Stars and Deeds. There is also a catchup mechanic that gives you an extra Era star if a few of the other Civs have already passed through the next Era.

Staying in your current Era will give you the option to build the Cultural wonders of that Era as well. You have to keep in mind that not progressing first has also it benefits as well.
Transcending your culture gives you more fame (as percentage ) and you get to keep your current features. That is also a viable strategy.

Some tips of you want to breeze through the Era and get as well as possible to a new culture :
- Check what the Era stars want you to do and focus on two of them. You will generally manage to easily hit a third or even fourth category just by gameplay.
- In the Neolithic Era you can split your tribes to cover more ground and reach more curiosities. Think of them as goody huts from Civ but they appear continuously.
- Units cost population and when you disband them, they add that population to the city. Thus you can disband some of the scouts you get in the Ancient Era to boost your population and assigned some Builders or Farmers straight away.
- Population unlike Civ do not work tiles, they can have one of four jobs.This is much akin to Specialists from Civ but here all your population are specialists. This makes buildings and bonuses which grant extra slots to a proffesion or extra yields all the more important. A granary in Humankind is has a bigger impact that a granary in Civ.
- Do not stress over the fact that the AI gets first to an Era. You can grab more fame from your current Era and then pick a culture that can still work with your overall plan. Unlike Civ this game does feel intended to be replayable as you will often get a different choice of cultures. Let me stress that transcendence is also a valid choice.

Playing my first game I also saw some opponents stuck with very low fame for a long time.

Normal difficulty here- some AIs are snowballing super hard and super fast. It seems to me whoever chooses Harappans is golden. Also I don´t understand how some AIs can hit ancient era like turn 6?

It is not easy to figure out the difficulty for this game. In general the AI is fairly tough or is aware of too much on the map. This game isn't one of those games you can just pick HARD, and wing it. That much is certain.

After playing several games I can certainly affirm to anyone that someone Fame Star rushing isn't necessarily going to curb stomp you other than building up a good score for the end game victory. Someone rushing fame doesn't necessarily have high tech, but they might, it's just not a given I want to emphasis here. Either way, ultimately this simply shows a gameplay strategy where rushing has its merits but it also has its flaws. And I can assure you an empire rushing fame is not building typically a competent military, think of them as AI's largely building empires like someone in Civilization just turtling and building Wonders. Go and be more aggressive to those players as they are not being pressured is basically how I've seen the game flow go with this now.

I consistently see the first AIs advancing to the Ancient Era after 6-8 turns. I have a feeling there's a hidden game function boosting some AIs in the Neolithic and possibly Ancient Eras, so not all civs advance equally. This would ensure that, by the Classical Era, you have both stronger civs to contend with and weaker ones to bully.

Maybe they only get +10 food on curiosities, instead of +5 :p

I just had a game where an AI was at 14.4k fame, next highest was 4k. Which is kinda cool I guess but I never even saw them before they Influence destroyed me with little to no counter. Love it but also ???

I've been having this 'issue' as well, particularly with the Ancestor AI, but maybe the game sort of picks an AI opponent to 'boost'. I've had it happen (in Town and Metropolis difficulty, at Normal and Slow speeds) early, mid, and late game, obviously with the late game one being the most frustrating. I'll be winning with a decent lead, sometimes just crushing it, and suddenly an AI opponent just starts slamming out Era stars and fame points at a pace I can't compete with, seemingly no matter how well I'm doing.

I can't seem to figure out how they do it either. I've seen them do it from a completely isolated position, to an all out war from nearly every side. They won't even be exploiting all the resources they have access to, I'm not getting notifications of them getting a ton of World Deeds, they're usually not expanding at a rapid rate, and they're not transcending as another poster suggested, so I'm truly at a loss. I've thought maybe the little bonuses AI get are just that powerful, but after inspecting them it just simply can't be the case. While some of them are strong, none seem so strong to explain how fast and sudden they pull ahead.

I've been really enjoying the game so far, but these seemingly unexplained great strides AI opponents can start taking out of nowhere has been one of the most frustrating elements so far. So if it's something that can be mechanically explained that I'm just missing, I would love to have it spoon fed to me lol

Yeah, it's really terrible design, they always stumble somewhere with their actual game mechanics and I guess this is it for HK. It's cool and all to give the AI some bonuses to account for the fact it can't reason like a human, but flooding them so hard like this disconnects the opponent from the actual game - it doesn't matter if you have strategic positioning on the map, certain resources, harass them in x way, because they get the vast majority of their income by virtue of not being a filthy player. Might as well call them aliens and have them build in the sky and come down in spaceships for all the relevance your interaction with them seems to have.

It's even more powerfully dumb since units cost population, but they don't really have that concern because hey free numbers. Very early aggression is massively punished because they can pump units without really sacrificing output, it doesn't matter if you ambush them early at all.

This has always been a feature of amplitude games - the mechanics are geared towards exponentiality, so that when someone is able to find a bit of leverage they can quickly sky rocket with it if they lean into it properly.  Often it's the human player doing this, but the AI sometimes stumbles onto a lucrative loop as well and can gain similar growth, especially with the help of it's innate bonuses.

This mode of play is a lot of fun, and one of the things that I like about Endless Legends and Endless Space - finding that edge to go exponential and seeing things rapidly accelerate can be really exciting, and is a fast-paced approach to turn based strategy that feels quite fresh. 

I don't think it makes sense for Humankind, though.   Having one nation speed through the last 3 eras in a handful of turns does not feel satisfying.