So I found this sentence:
弥生時代にはいると食事の内容が大きく変化しました
I understand its meaning, something like
In the Yayoi period, the food content changed a lot
However, 'In the Yayoi period' is a free translation, because I don't really know what いると means. Does と work as 'when'? How would you translate that? A literal translation would also help for better understanding.