Perhaps the change that has the most impact when moving from a hunter-gatherer existence to one based in agriculture is the simple fact that agriculture generates a surplus. A hunter-gatherer tribe generates roughly the amount of value in a year that they consume, whereas with agriculture a group of farmers can generate much more value than they consume in the same period. Once you move from an agrarian society to an industrial one, the ability to generate such a surplus increases.
From a simplified macro-economic perspective, the reason for this is quite simple. A hunter-gatherer tribe has little left over for positive investment, they create enough value to feed themselves, and cover replacement investment (fixing their weapons, clothing and equipment) but they do not generate enough to improve their ability to produce. An agrarian society generates enough value to feed themselves, clothe themselves and invest in things that expands their capacity to produce. An industrial society generates enough to cover their basic needs and invest greatly in productive capacities with leftover resources for excess consumption.
Utilizing the simplified C-I-S model for a nation, where C stands for consumption, I for Investment and S for Saving, we could argue that theoretically for a hunter-gatherer tribe 90% of their generated value goes to consumption, and 10% to investment. For an agrarian group, perhaps 75% goes to consumption, 15% to investment and 10% to saving, and for an industrial population 50% is consumed, 35% invested and 15% saved. These are not empirical numbers, but serve to symbolize the development of human economics.
Gross investment is composed of net investment which is spending that increases the availability of fixed capital goods, means of production and goods inventories. For instance, buying a new building, new machine or increasing your inventory of products, and replacement investment which is replacement of depreciated capital goods. As an example, buying a new house is increasing the availability of fixed capital goods, maintaining that house over its lifetime is a case of replacement investment, as you are fixing the wear and tear of use.
One of the key elements is that if investment falls below replacement investment levels, then the ability of a group to produce goods and services goes down. Put very simply, if a two man carpentry crew has 2 hammers, and break 1 hammer a year on average, but do not make enough money to replace that hammer, their productivity will decrease.