The first goal of the project, known as the Open Tree of Life, is to publish a draft by August 2013. For their raw material, the scientists will grab tens of thousands of evolutionary trees that are archived online. They will then graft the smaller trees into a single big one.

These trees represent just a tiny fraction of all the known species on earth. The rest are classified in the old-fashioned Linnaean system, in which they are assigned to a genus, which is assigned to a family, which is assigned to a kingdom, and so on. Dr. Cranston and her colleagues will use that information to put them on the tree as well. All the species in a genus, for example, will belong to branches descending from the same common ancestor. The Linnaean system will give the tree only a rough picture of the true relationships among species.

“Parts of it will be quite good, and parts will be quite bad,” Dr. Cranston said.

She and her colleagues will then enlist the entire community of evolutionary biologists to make the tree more accurate. They will set up an Internet portal where scientists can upload new studies, which can then automatically be used to revise the entire tree.

Even as the tree of life comes into sharper focus, it will continue to grow. Each year scientists publish descriptions of 17,000 new species. How many species there are left to discover is an open question; last year a team of scientists estimated the total to be 8.7 million, although others think it could easily be 10 times that many.

When scientists publish the details of a new species, they typically compare it with known species to determine its closest relatives. They will be able to upload this new information into the Open Tree of Life. Scientists who extract DNA from the environment from previously unknown species will be able to add their information as well.