Broadly speaking, Ward's idea is to expand the tree of life upwards, adding a level of classification above the current highest level, domain. This next level he calls dominions – the dominion "terroan" would include life of Earthly origins; another he calls "ribosa" would include life based on RNA. Further dominions, he suggests, could be formed to cover life that might have a different base altogether.Of course, the problem here is that even in the case of life with very Earthly chemistry, there are variations which seem to be just as plausible as what we have now. For example, we use ATP to store energy. Any of the other nucleotide bases could have been modified to store energy: TTP, GTP, or CTP are just as good for storing energy. It's just that one common ancestor came up with ATP, and its descendents filled the biological niche before any competition could get started.
Yet a "dominion" that used CTP to store energy is certainly life other than as we know it.
In fact, Ward points out, scientists are already working on developing life forms that don't fit current classification systems. For example, he mentions microbes being synthesized with at least one amino acid more than the standard 20. "To me, that's alien life," he says. "There's no place in the current classification system to put this stuff."How about silicon-based life, Outsiders, Mesklinites, and other, really alien life?
Higher up again, beyond the level of domain, he proposes a classification that includes different "trees of life" themselves, called arborea. "We've not yet run into a different tree, but we're going to start making them soon," he adds.