To the degree that "human nature" involves the way we behave due to environmental factors (both in development and epigenetics and current stimuli response), I think we have, on average, changed. Probably not enough to justify your statement being false, though, in this context.
But with more food, security, less stress, less trauma in early childhood (and even the womb), you actually do change the "human nature" of adults, if by "human nature" we mean the statistical probability a person will act in one way or another in response to certain stimuli. Think about it: all the people who go through our criminal justice system who grew up poor, with crappy parents who beat them, etc.--they are the minority today. 1000 years ago, they were the vast majority of people.
For an interesting discussion of some of these topics, a timely interview:
Also, Sapolsky's book
Behave provides a deep dive into the science behind causal mechanisms of human behavior. From the neural responses and the chemistry behind it that cause our actions in the moment, to the evolutionary mechanisms that caused these neurons to exist. I highly recommend it, even if you--like me--want to hold on to the notion of free will with every fiber of your being.