I think the Land Ethic makes a lot of sense. It's not perfect, but it takes a very holisitic approach to ecology which is a good thing. I would say that ethics did start out as a completely human-centric idea, but as we advance, it might be time to change that. Yes, as any other species, we tend to look out for our own. But humans also have become intelligent enough to have a conscious impact on many other organisms, and practically the entire planet.
Technology and exploitation of resources have only recently began to have a real impact on the world. In medieval times, you could argue that the tiny, unstable population of humans could not have a huge impact. Now there's 7 billion of us and we are growing fast. We've been making other species extinct, polluting, depleting resources (which are vital for our survival too, by the way). I think should we try to act responsibly and rationally to preserve finite resources and reduce our negative impact on the planet to a reasonable level. That is not to say we should stop our growth. Just manage and plan for the future better. Of course, when it comes down to it, I think we will always defend humans before anything else, and that is what our ethics is grounded in. (Due to the fact that we initially learn to have compassion for people close to us.)