New species evolve whenever a lineage splits off into several branches and so a common metaphor for evolution has been to describe evolution as a 'tree of life', where every branch constitutes a species.

But since about 99.999% of all species that have ever existed are already extinct, and we have never even known about them, the tree is more like a bush with things constantly growing and falling off. Last year, a consortium of some hundred researchers reported that the relationship between all major bird clades had been mapped out by analyzing the complete genome of around 50 bird species. This included the exact order in which the various lineages had diverged.

For students, the start of the school year means new classes, new friends, homework and sports. It also brings the threat of head lice. The itch-inducing pests lead to missed school days and frustrated parents, who could have even more reason to be wary of the bug this year. Scientists report that lice populations in at least 25 states have developed resistance to over-the-counter treatments still widely recommended by doctors and schools.

The researchers are presenting their work today at the 250th National Meeting&Exposition of the American Chemical Society (ACS), the world's largest scientific society. The meeting features more than 9,000 presentations on a wide range of science topics and is being held here through Thursday.

Dosing obese teens with vitamin D shows no benefits for their heart health or diabetes risk, and could have the unintended consequences of increasing cholesterol and fat-storing triglycerides. These are the latest findings in a series of Mayo Clinic studies in childhood obesity.

Seema Kumar, M.D., a pediatric endocrinologist in the Mayo Clinic Children's Center, has been studying the effects of vitamin D supplementation in children for 10 years, through four clinical trials and six published studies. To date, Dr. Kumar's team has found limited benefit from vitamin D supplements in adolescents. The latest study, Effect of Vitamin D3 Treatment on Endothelial Function in Obese Adolescents, appears online in Pediatric Obesity.

The results of a recent study show that children who report eating more polyunsaturated fatty acids (PUFAs), found in tree nuts, seeds and fatty fish, and consume a higher ratio of PUFA: saturated fatty acids (SFAs), have more lean body mass, lower percent body fat, and less intra-abdominal fat (belly fat).

The study was published in "The Journal of Nutrition" and conducted by researchers at the University of Colorado Anschutz Health and Wellness Center and the University of Colorado School of Medicine at the Anschutz Medical Campus in Aurora, Colo. in collaboration with the University of Alabama at Birmingham.

For more than a century, researchers have tried to pin down exactly why so many animal species play in their infancy. Now a new study in wild macaque monkeys has found that infants who play more actually boost key motor skills. However, these skills are acquired at a cost. The researchers also discovered that active infants grow more slowly.

So what are the evolutionary reasons behind this trade-off? And should human parents who want tall children sit them in front of the TV rather than letting them play in the garden?