If I understood your question correctly, the reason why we're moving differently than people in ancient history is that we have new available technologies which enable us to transport from one city to another via car, train, or airplane. None of these were available during ancient times. What remains the same, however, is that we still walk by foot for shorter distances, similar to our ancestors.
The correct answer to this open question is the following.
As it happens in most cases, this issue has advantages and disadvantages. But the thing is that under the present economic, political, and social conditions, smaller families have better chances to offer the support to raise children and give them education, shelter, proper nutrition, and better chances to thrive.
In the past, natality control was not an issue. Parents could have two, three, and more children. The economic conditions were completely different from the ones we are living in today.
Education has played a major role in creating consciousness in new generations about having a small family or not at all. Young couples have different priorities such as personal development, scale-up in the corporate world, travel, and other different options.
The answer is trade. So it is important for other countries to exchange currencies so that goods from other nations can be traded.
