Since ancient times, scientists and philosophers have pondered the fundamental question of what makes some organisms male and some female. Some hypotheses were ... are important for human health. A ...

All species on Earth, both living and extinct, are related. We know this because of a biological tool called the tree of life. This "tree" takes the form of a diagram that maps the relationships ...

By combining data on all known bird species, this tree is the most impressive snapshot yet of one of Earth’s most awe-inspiring classes of animal.