God named them “Mankind” when they were created

Genesis 5:1-17
When God created mankind, he made them in the likeness of God. He created them male and female and blessed them. And he named them “Mankind” when they were created.
When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth. After Seth was born, Adam lived 800 years and had other sons and daughters. Altogether, Adam lived a total of 930 years, and then he died.
The Bible often provides family trees similar to this one to define an important line of descent. Many of them are selective, sometimes in order to create a pattern of a certain number of names. We are not meant to add up all the figures and work out the length of time covered. Ussher did this, arriving at 4004 BC for the date of Adam. Yet we know, from Jericho and other cities, of a civilization going back to 7000BC, and that is not the beginning of human history.
In the ancient Near East numbers were often used to signify importance, rather than actual quantity. The reducing ages from Methuselah to Joseph may be meant to suggest the cumulative effects of sin. Enoch's life of 365 years, a completed year's cycle of days, is worth special note.
Why?
Adam and his sons and grandsons lived a longer life than we are.
The reducing ages may be meant to the cumulative effects of sins.
How?
Our life is determined by the Lord, longer or shorter is not so important, but the living meaning is more important.