Answer:
The key thing to consider here is that water's expansion when freezing is what allows ice to float. The fact that ice remains on top of water is what prevents lakes and rivers from freezing solid. The layer of ice on top serves to insulate the water underneath from the cold air, meaning that life can continue to survive in bodies of water even if they're totally iced over.
If that were not the case, and ice sank, many of the planet's lakes would not be able to support life in the winter. The polar ice caps would also be very different. Whereas the Arctic is a layer of ice floating on top of water, sinking ice would turn it into a giant ice cube extending all the way to the ocean floor. The Antarctic would experience an encrusting of ice all around the continent. Neither of these would be conducive to life.
So, in short, the world would probably support dramatically less life than it does now. Failures of rivers to flow in winter would likely mean that most regions beyond the subtropics would be nearly barren. People would likely have been constrained to the tropics. The oceans would be much lower due to so much ice being tied up in the poles, and there would probably be much less rainfall. This may have made the development of agriculture as we know it impossible.
So, if there were people at all in such as world, they'd probably still be hunter-gatherers, without civilization as we conceive it
