The key verse to understanding Ecclesiastes is
- 1 : 3end - 'under the sun' [most versions].
The majority of the thoughts put forward are from an earthly (the fallen world) not a heavenly perspective. How else could 1:2 be read?
- "Meaningless! Meaningless!” says the Teacher. “Utterly meaningless! Everything is meaningless.” [NIV; BibleHub]
The original was certainly totally inspired by God in the sense that it was written exactly as He wished. But it puts forward, as you say, the thoughts of a (reasoning) man not relating at most points to God. Why did God sanction its inclusion in the canon? To help those who find themselves wallowing in the same mental swamp the author (very likely Solomon) did. But it is vital to read the book in context.
As 2 Peter 1:20 Berean Literal Version; BibleHub says:
- knowing this first, that any prophecy of Scripture is not of its own interpretation.
While this verse itself is interpreted in various ways(!), a valid one is:
'Don't base a doctrine on a single verse, and don't skew the context.'
Philip Yancey, in 'The Bible Jesus Read', addresses the concepts here very eloquently (though there may be no quick way out of the sense of futility described).
There are some gems, intense rays of light, hidden among the Godless views taking up most of the book.
As for the worth of a man: God's view is spelled out in Matthew 26ff, and no more powerfully addressed than in Father sacrificing his own Son for all mankind.
You can find verses showing that (non-nasty) animals will still exist in the New Earth, when New Heaven and New Earth are in sync. But note that 'bara', the Hebrew for create (as opposed to [then go on to] mould), is used 3 times in Genesis I:
- Space / time / matter (verse 1)
- Sentient life forms not including man (verse 21) (possessing souls: intelligence; emotions)
- Spiritual life on Earth (vv 26-7) (Adam and Eve) (capable of relating to God)
Man is not a highly-evolved animal.