Early, weak and defenseless humankind survived by adopting primitive communism. These early small hunter-gather societies shared everything equally, among every member, including knowledge, and rely on one another equally, in order to survive. There was no leadership. No other animal had ever developed such a system of equality. Perhaps this is why humankind was able to survive and develop much more, especially its brain, compared to all the other animals. The following is a link, long, but worth reading...
Modern studies of surviving hunter-gatherer and early agricultural societies have upheld the essential core of Engels' account. Hunting-gathering peoples live in what are usually called 'band societies'--based on loose knit groups of 30 or 40 people which may, periodically, get together with other groups into bigger gatherings up to a couple of a hundred strong. There is no formal leadership, let alone class division within these societies.
http://pubs.socialistreviewindex.org.uk/isj65/harman.htm
Another link that discusses the Kibbutz system in Israel...
It has been argued, however, that an unstratified society is a possibility. Such a society, in which everyone is equal, is known as Egalitarian. The Kibbutz system in Israel is a good example of how this could work. Roughly four percent of the Israeli population live in one of the country's two hundred and forty Kibbutzim. The Kibbutz system was established in 1948 when the Jews reclaimed Israel from the Arabs, and was an attempt to escape from the individualism and competition that prevailed in most of the developed world. It is run according the Marxist principle-"to each according to need, from each according to ability".
Most kibbutzim are agricultural communities with a population ranging between two hundred and seven hundred people. All property, including land, buildings and produce, is communally owned between the members of each kibbutz. All commodities are freely distributed to members as and when they are needed, and all services are freely available. Children are all brought up and educated communally. There are no economic differences within kibbutzim as money is not used and general assemblies are held to make any major decisions. This may sound like an unstratified society, but Eva Rosenfeld, who studied a kibbutz in 1974, argues that there is some stratification.
Her studies concluded that there were two distinct strata within kibbutzim- "Leader/ managers" who were elected to run the kibbutz and to allocate jobs and "Rank and file" who undertook these jobs. Although the leader/ managers did not have any economic advantage over the rank and file, their jobs earned them more prestige. She found that there was an uneven distribution of "emotional gratification"- there was stratification but, unlike western societies, it was based on power and prestige rather than material wealth.
Rosenfeld's findings show that, even in a society with no economic differences, there can still be social stratification. Within the kibbutz system there is comparative equality but it is not truly unstratified. It is possibly the closest thing there is to an unstratified society in modern times. Tribal societies could also be said to be classless but, even in these situations, there are differences between individuals based on their status within the tribe- there are usually leaders and followers.
Social hierarchy can be based on the possession of different qualities in different communities, and some societies are more equal than others, but none can ever be completely unstratified as there will always be inequalities in certain areas, due to individual differences.
http://www.hewett.norfolk.sch.uk/curric/soc/lowerSixthA.htm