Why did people in biblical times live for so long?

Question

The Tanakh records many instances in which people lived exceptionally long lifespans, some even nearing 1,000 years old.

There tends to be a trend in the recorded ages, in that the earlier generations recorded in the first chapters of the Book of Genesis lived for a long time, and as the generations went on, lifespans became shorter. For example: Adam lived to be 930, Abraham 175, Moses 120, and King David 70.

While throughout history the average lifespan has fluctuated back and forth depending on the era, humans tend to live no more than around 70 years, on average. (Longer lifespans are of course, possible, but they are outliers in the data.) What about the early generations enabled them to live for so long? What do traditional Jewish sources have to say concerning the long lifespans recorded in the Tanakh?

Answer 1

A little kabbalistic/deep, but refer to the Or HaChaim on Bereishis 47:29:

Originally, G'd assigned great tasks to the antediluvian generations and at the same time equipped them with commensurately greater souls, each one of which consisted of many "sparks." He assigned a single day to each "spark." This is alluded to in Exodus 16,4 when the Torah speaks about people collecting the manna on a daily basis, i.e. דבר יום ביומו. The word יום may be understood as ענף, a branch or sprout of each soul. G'd granted each human being a certain number of days, i.e. He allocated to them approximately 300.000 such gemstones as mentioned in the parable. After the failure which resulted in the deluge, all of this was restructured in order to help us achieve what is expected of us in a lifespan averaging seventy years.

The basic idea of this is that prior to the sin of man, the soul consisted of a lot of sparks. There was a lot of "work" to do in this world, and hence, as the Or HaChaim says, the lifespan was much bigger than after the flood.

Rabbi Dovid Rosenfeld wrote an article on this topic and explains that:

[...] when man became sinful, G-d began shortening his lifespan – in a gradual process which was not completed until the time of the Exodus. G-d did this so that man would be more strongly aware of his mortality. He would be forced to live his years knowing his soul would soon be returned to his Creator for judgment, and he would have to live with that awareness throughout his life.

This Chizkuni is also interesting, although it does not answer your question, it does deal with the subject of lifespan being shortened after the flood.

Answer 2

Maimonides suggests that even in biblical days long life spans were unusual and may well have been isolated "miracles":

About the length of man's life in those days [before the Flood], I say that only the persons named lived so long, while other people enjoyed the ordinary length of life. The men named were exceptions, either in consequence of different causes, such as their food or mode of living; or by way of miracles, which admits of no analogy. [Rambam, Guide for the Perplexed 2:47]