Why does the dinosaur let the wounded dinosaur live?
Yes, we are all fascinated (or abhorred) by the inclusion of a few CGI dinosaurs in Terrence Malick's 'Tree of Life'. Personally, I enjoyed the dinosaurs thoroughly, but...
Why did the predatory dinosaur (Wikipedia says Ornithomimus) let the wounded Parasaurolophus live? Wikipedia says the Ornithomimus:
...places its foot on the Parasaurolophus' neck, preparing for the kill, but then reconsiders after watching it struggle.
In other words, Wikipedia suggests the dinosaur committed an act of mercy. But can we be sure that the predatory dinosaur didn't let the injured dinosaur live for some other reason, perhaps because the injured dinosaur was sick? Could the 'mercy' in fact have been an act of self-preservation? Something else?
To my understanding, this scene is a comment on the opening line and one of the central themes of the film:
A man's heart has heard two ways through life: The way of nature and the way of grace. You have to choose which one you'll follow. Grace doesn't try to please itself. It accepts being slighted...forgotten...disliked... It accepts insults and injuries. Nature only wants to please itself.
We would assume--and have been shown until this point in the evolution story--that nature is the only mechanism at work in the non-human world. But then we see a dinosaur, that symbol of lustful hunger and violence, performing an act of mercy. Malick is pointing to a turning point in evolutionary development. "Here is grace, this is where God is born. Not in the heart of man but long long before that."
And if that's the case, then perhaps looking for grace in the "purely human qualities" is wrong. Perhaps the more profound contact with the "way of grace" is through some sort of atavistic rejection of rationality, beauty, truth or whatever else you perceive to be human.
It's a pretty rich moment and I think you could probably find a lot more that it's hinting at but it's pretty clear that it's meant to be read as one of a series of meditations on grace vs nature.