In the last stanza of the poem the poet mentions

Their logs held tight in death’s still hands
Was proof of human sin.

From the two lines above it is clear that the ‘human sin’ here is the selfishness of the human race. They are not only cruel to other animals on earth, but also bear discriminatory attitude towards the fellow men over petty barriers like class, caste, colour, creed, religion and so on. Though they all had a log of wood to keep the fire burning and help each other survive the cold weather, they chose the other way around and accepted their death, holding the logs tight in their hands. These prejudices are not good for the human civilization or at least for their survival in the longer term. So, the poet calls it ‘human sin’.

In short, selfishness is the ‘human sin’ here. And the logs in their hands are the ‘proof of human sin’.