I'm looking for a (rough) estimation about who owned the productive capital since the beginning of history.
I'm mostly interested up until before the industrial revolution.
For example, in an agricultural civilization, the ownership of the productive capital (tools and fields) could be either concentrated in a few people (Landlords vs Slave) or equally distributed (small owners cultivating their own land).
Is there such data available ?
This varies with the culture - in ancient Egypt and Babylon the temples claimed ownership of the land, but the peasants had rights to their plots. You will find something similar in all very hierarchical systems. If the society is flatter, ownership may be communal, with the individual plots assigned annually by the village elders, or by some heredity rule.
Note that in none of the above societies could the peasant alienate (sell) the land. Similar societies continued to exist down to and well into the first millennium BC.
OTOH, within the urban environment the rental, sale, and purchase of land and buildings, by individuals, is well documented before 1500 BC.
With the introduction of coinage, in the 6th century 600 BC, small scale transactions were facilitated, and land holdings could be monetized.
As a result you see the development of a "middle class" of small land holders, often with mortgages, marked by stones on the boundaries of the land to indicated the debt. Some even borrowed against their own person; indeed, money is the source of many evils!
So if they could keep out of debt, and pay their taxes to the state, these middle class landowners could thrive, as they did at Athens and Latium. The tendency, of course, is for the person with more capital to buy up more land, concentrating it in the hands of the more wealthy. But even in the late days of the Roman Empire, there were still plenty of small free-holders in Italy.
It requires peace and prosperity for the free-holder to exist; in times of trouble they would sell out, or bond themselves to the local lord, who offered protection via his private militia. This was a common event in late Roman times, and even after.
This is not really an answer for your question, but perhaps a guide as to how to search the histories for an answer.
I think this is a very legitimate question, though it is probably not possible to answer it due to the paucity of statistical material in the pre-modern period. But it is possible to pose it from a theoretical angle. Following up on the work by Pareto about land ownership in Italy at the end of the 19th century there is a view that there is a stable mathematical relationship governing the distribution of wealth in any given country. You make a list of (let us say) the 50 richest families in the country and number them from 1 to 50. Then you write down how much capital each family owns. The surprising fact is that the rank number of each family will be inversely proportional to their wealth; that means that the 2nd richest family will be half as rich as the wealthiest, the 3rd richest will be one third as rich, the 4th richest will be one quarter as rich, and so on. If you make a graph putting rank on the x-axis and wealth on the y-axis the result will be a regularly descending parabola; if you map it on a double logarithmic axis the result will be a sharply descending straight line. This is what in mathematics is called a power relationship. It is believed (by some) that this is a law of nature, independent of human will.
Here is a classic study by a famous mathematician: https://www.jstor.org/stable/2525289?seq=1#page_scan_tab_contents
