What could have caused mankind to have lived such LONG lifespans before the worldwide flood? The average lifespan before the flood (ten generations) was 857 years. After it the average life span (for the ten generations from Shem to Abraham) was only 317 years! Many who refuse to believe in the Bible say that the ages upon death found in the first several chapters of Genesis are nothing but fabrications and tall tales.
Several things could have contributed to the shorter lifespans after the deluge. Scripture denotes that the waters God brought on the earth would destroy not only animals by the earth itself.
"And God said to Noah, 'The end of all flesh has come before Me, for the earth is filled with violence through them. And, behold, I will destroy them with the earth.'" (Genesis 6:13, Holy Bible in Its Original Order - A Faithful Version (HBFV))
"'And I will establish My covenant with you. Neither shall all flesh be cut off any more by the waters of a flood. Neither shall there any more be a flood to destroy the earth.'" (Genesis 9)
The earth changed dramatically after the great flood. The differences included the altering of the climate, atmospheric changes, changes in the hydrologic cycle, geologic features, a significant increase in harmful radiation reaching ground level, man's dietary habits went from solely plant-based to one where meat was eaten, and so on. These and many other factors led to man's much shorter lifespans.
The first century historian Josephus asserts that pre-flood man did live very long lifespans. He attributes their longevity to God's mercy and the fact that the food they ate was much better at extending their years on earth than it was after the deluge (Antiquities of the Jews, Book 1, Chapter 3).