Modern atheism is only as sustainable as the concept of 'humanity' is. If we take man to be an animal - simply one among many - then perhaps atheism would be as honest as it claims to be. However, in atheism, the residue of metaphysical dualism survives precisely in God's ghost. Because there is no entity with a personality and a list of preferences for mankind's conduct, atheism must return to the Logos under a new sign and symbol, whether that be 'humanity,' 'love' or 'survival.' It is always assumed that humans are better at at least one of these than other creatures and therefore the best animal.
Even keeping it at the level of the 'important animal' proves to be unsustainable. Imagine that if we could, for a moment, assume that man is not the most important animal, then what possibly could be? We quickly discover that we have no frame of reference for what makes something 'most important,' as humans cannot even all agree on what constitutes justified killing. The conclusion is always nihilism in the end, which causes one to throw out the notion of importance altogether in exchange for a completely different metric; namely power.
Power, after all, can at least be located to a measure of identifiable change. Only the most foolish man would try to convince others that power in this universe ends with man. Even those forces which man does not see as possessing any kind of agency have more power than him - power to wipe him out in minutes if any number of variables were slightly off (or on, for that matter).
The pre-Socratics were in a similar position to modern atheists but couldn't bring themselves to think so highly of their own agency (even though they valued it more than that of the gods). The gods were, to them, something to be viewed with great suspicion. The peasants worshipped them, praised them when things went well and lamented when things didn't, but the great thinkers were never satisfied with this formula. They knew that it was better that the universe be indifferent to them than to do something to earn the attention of the divine.