Biology is literally the study of life and so, it is the broadest of the life science fields. Subsections of biology include biochemistry (the study of how chemical processes work in living organisms, such as how respiration works) and evolutionary biology (the study of how evolution has caused organisms to change throughout time). Students writing a dissertation in biology would likely focus on a specific subsection of biology, as well as a specific organism.
Besides biology, human health is another large subsection of the life sciences. While the life sciences do not include aspects of health that are primarily intellectual (such as psychology or educational development), it does include almost all physical aspects of health, such as human genetics or developing new medicines. Indeed, medicine development is an important aspect of health science, and thus a student involve working on their Ph.D. may be involved in running trial experiments to prove or disprove the effectiveness of a newly developed drug.
Ecology is the study of how different organisms coexist with each other and their environment in a community, and thus it is a life science study at a larger scale than biology or health. Ecology has become increasingly important as human beings try to gauge their impact on natural communities. One possibility for a Ph.D. topic in ecology would be to take a select area that has not been developed with buildings and study its plants and animals in-depth.
Many Ph.D. programs (including ones at Harvard and Northwestern) offer students the possibility to work on dissertation topics that are interdisciplinary within the various fields of the life sciences. Thus, for example, a student working on such a project could study how a certain insect evolves in an environment that has not been proliferated with human activity, versus one where human beings have altered the environment; this project would combine evolutionary biology and ecology. Such possibilities are endless in the life sciences because no topic truly stands alone when it comes to dealing with living organisms.