From
How Sacred is Life?
by Roger Caldwell, Planet 125:

According to Descartes there was a radical division between mankind and the animals. Man was a kind of composite creature, part bodily and material, part spiritual and immaterial. Animals on the other hand, were bodies only: they were a kind of natural automata, moving machines that could neither think nor feel, as lacking minds. If man was both mind and body it was necessary that there be a link between the two, and Descartes famously located it, with a somewhat cavalier gesture, in the otherwise innocuous pineal gland. (Quite what function he thought it possessed in other animals where it is equally evident remained unclear.)

Implausible as it is - and it must be noted that it seemed implausible as well to a good many of Descartes' contemporaries - the Cartesian paradigm has retained its dominance for more than three centuries. We are still led to think in Cartesian terms, for all that, even in the seventeenth century, there were perfectly respectable alternatives, such as Spinoza's mind-body parallelism which involved no such rational disjunction betwen man and animal. Descartes' paradigm succeeded for a variety of reasons, a major one being that it fitted well with Christian orthodoxy whereby God held a special dispensation over mankind: Man, with his immortal soul, was put on earth to serve God's purpose, all the other fauna and flora were there to serve man's. This is preserved by a society less certain of the soul's immortality but nonetheless sufficently haunted by the vestiges of the Judaeo-Christian tradition to retain the dogma of the sacredness of human life....

...If ethics is to be, not a God-given pronouncement of the sanctitiy of human life, but a device for the preservation of the species, this doesn't mean - far from it - that anything is allowed, but rather that only that is allowed which, on the evidence available, is likely to go against that purpose. In this recognition it will be necessary to take into account that human life depends on non-human life, that for us to survive as a race there must be a limit to the numbers of its individuals if the natural resources of the planet are not to give out. The more ruthlessly those resources are tapped the less viable our future as a race seems, and the lower the quality of the life of the individual is likely to be. Given our melancholy record it appears, for example, that all surface rocks on earth are contaminated with the radioactive fall-out from nuclear tests. We need to recognize too that we have in effect become, in the rather grandiose phrase of Heidegger, shepherds of Being, a rôle we have not so far shown ourselves well-qualified to play.