Agriculture
The period of human history in which we have had a significant impact on the planet is today called the Holocene Era. This period begins about 12,000 years before the present. The mainstream view is that humans invented agriculture which then gradually spread around the world. Of late feminists have been fond of claiming that women invented agriculture.
An alternative view, which is supported by the evidence, is that humans always knew how to grow crops but didn't do so because there was no reason to put in the back-breaking work. Children watch seeds sprout. It isn't a huge leap to realise that planting a seed will result in a plant growing. Even today some hunter-gathers deliberately plant seeds as they move knowing that they can return and eat food from these plants later. It is highly likely that ancient hunter-gatherer societies did the same.
Hunter-gathers today work the same or fewer hours per day than settled communities, and that's when we take in to account that modern hunter-gathers live on marginal land. Before agriculture hunter-gathers lived on the best land. The archeological record indicates that hunter-gathers were taller and healthier than the agriculturalists that came after them. These both likely occured because hunter-gathers had a more varied diet. Hunter-gathers eat what is available in the environment while farmers plant foods suitable for growing. Hunter-gathers lived longer healthier lives than agriculturalists. The period between the widespread use of agriculture and the industrial revolution can be seen as a transitional period. It was perhaps the worst time to be alive as a human. Living as a hunter-gatherer would have been good. Living in an advanced industralised society also has many benefits. But the people who lived between these two periods were perhaps unlucky.
Hunter-gathers were nearly as tall as people who live in industrialised societies while agriculturalists were shorter. With the advent of the industrial revolution humans have for the first time surpassed the height of their ancient hunter-gatherer ancestors.
The manner in which humans left agriculture in large numbers at the start of industrialisation is further proof of the difficulty of a farming lifestyle without modern mechanisation. This happened in England 250 years ago and it's happening in China now.
This raises the obvious question, if being a hunter-gatherer was so good why did agriculture appear at all? One hypothesis proposes that humans only started farming because they had to. Agriculture has one redeeming feature - it can feed far more people per unit area than hunting and gathering can. In this sense agriculture is a trap. Once a society adopts agriculture the population explodes and they can't revert to a hunter-gatherer lifestyle.