A good way for a tech tree would be discovery. Lets say there is a lightning storm and there is a fire. The sapien will discover that the fire keeps them warm. They will want to create fire. You can tell them to create a camp fire and it will take a while at first. Them banging rocks and sticks together. Eventually they create fire and they know how. They teach other sapiens how. This will increase their engineering and knowledge skill. In another situation a sapien will pick up a sharp rock to hunt rather than a dull round rock. It will discover that hunting with sharp objects is easier. The sapien will then want to hunt like this even more and and will sharpen rocks. Then they eventually create spears and so on. This will increase the sapiens engineering, knowledge and weapons skill. Discoverys like this will be random but fairly often. A function like this i would assume is hard to implement but it would make the game much more detailed, realistic, and challenging. Again I understand if this might be out of the question because a feature this big would be hard to put in the game, but it would be really awesome.
I fully endorse this suggestion and would recommend the same system.
If a nearby sapien witnesses a complex action being executed then they have a chance of learning it based on some intelligence variable which they possess.
What started out as one character randomly stumbling upon something evolves into a whole tribe that is now better at this action.
I have plenty of other ideas but will try to compile them all into new posts.