The philosopher Whitehead said that all philosophy is a footnote to Aristotle and Plato and he had a good point. There are not many new ideas in ethics — most theories are in some ways a revising of old ideas and the debates today would have been recognised by the ancient Greeks. Not much is new.
However, there has been an emerging challenge to traditional ethics which is not fully recognised or articulated and which strikes at the heart of all traditional religious ethics.
In some ways this challenge stemmed from Ludwig Feuerbach who argued that human beings are simply animals — 'we are what we eat', we are simply material beings who copulate, give birth, grow and die. Life has no transcendent purpose and, essentially, no meaning except that which we create.
Darwin's discovery of the means by which evolution takes place (the survival of the fittest) as well as the work of Freud, Marx, Schopenhauer, Nietzsche and 20th century atheists such as Bertrand Russell, A. J. Ayer and others built on this insight. However it is the Australian philosopher, Peter Singer, who has done more than any other to codify and express this insight in ethical terms.
Singer is what is called a preference utilitarian — he holds that the more sentient beings can exercise choices and not suffer the better. He is passionately committed to the view that traditional religions such as Christianity, Judaism and Islam are speciest in holding that human beings are in an ontologically different category to animals and therefore argues strongly for the rights of animals. A dolphin or a monkey is able to suffer to a greater degree and also to experience happiness than someone with advanced Alzheimer's disease and, therefore, it is wrong to deny them rights. He is a vegetarian and a passionate complainer for animal rights.
Traditionally Christians have wanted to hold that humans were in a different category than animals — indeed it was a heresy called Traducianism in the early Church to hold that a man and woman could have sex and make a human baby — as this would have meant we were no different from animals. God was needed to implant a soul as the soul was immortal and could not be generated by human agency.
The early Church fathers such as Augustine held that souls were implanted 40 days after the conception of a boy and 90 days after conception