This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Attention-deficit hyperactivity disorder is the most common developmental disorder in children, affecting anywhere between 3-5% of the world’s school-going population. As the name suggests, kids with ADHD are hyperactive and easily distracted; they are also forgetful and find it difficult to control their own impulses.
While some evidence has suggested that ADHD brains develop in fundamentally different ways to typical ones, other results have argued that they are just the result of a delay in the normal timetable for development.
Now, Philip Shaw, Judith Rapaport and others from the National Institute of Mental Health have found new evidence to support the second theory. When some parts of the brain stick to their normal timetable for development, while others lag behind, ADHD is the result.
The idea isn’t new; earlier studies have found that children with ADHD have similar brain activity to slightly younger children without the condition. Rapaport’s own group had previously found that the brain’s four lobes developed in very much the same way, regardless of whether children had ADHD or not.
But looking at the size of entire lobes is a blunt measure that, at best, provides a rough overview. To get an sharper picture, they used magnetic resonance imaging to measure the brains of 447 children of different ages, often at more than one point in time.
At over 40,000 parts of the brain, they noted the thickness of the child’s cerebral cortex, the brain’s outer layer, where its most complex functions like memory, language and consciousness are thought to lie. Half of the children had ADHD and using these measurements, Shaw could work out how their cortex differed from typical children as they grew up.
The turtle’s shell provides it with a formidable defence and one that is unique in the animal world. No other animal has a structure quite like it, and the bizarre nature of the turtle’s anatomy also applies to the skeleton and muscles lying inside its bony armour.
The shell itself is made from broadened and flattened ribs, fused to parts of the turtle’s backbone (so that unlike in cartoons, you couldn’t pull a turtle out of its shell). The shoulder blades sit underneath this bony case, effectively lying within the turtle’s ribcage. In all other back-boned animals, whose shoulder blades sit outside their ribs (think of your own back for a start). The turtle’s torso muscles are even more bizarrely arranged.
This body plan – and particularly the odd location of the shoulder blades – is so radically different to that of all other back-boned animals that biologists have struggled to explain how it could have arisen gradually from the standard model, or what the intermediate ancestors might have looked like. Enter Hiroshi Nagashima from the RIKEN Center; he has found some answers by studying how the embryos of the Chinese soft-shelled turtle (Pelodiscus sinensis) shift from the standard body plan of other vertebrates to the bizarre configuration of adult turtles.
By comparing the embryos to those of mice and chickens, Nagashima showed that all three species start off with a shared pattern that their last common ancestor probably shared. It is only later that the turtle does something different, starting of a sequence of muscular origami that distorts its body design into the adult version.
Be it in sports or comedy, they say that timing is everything. In evolution, it’s no different. Many of the innovations that have separated us from other apes may have arisen not through creating new genetic material, but by subtly shifting how the existing lot is used.
Take our brains, for example. In the brains of humans, chimps and many other mammals, the genes that are switched on in the brain change dramatically in the first few years of life. But Mehmet Somel from the Max Planck Institute for Evolutionary Anthropology has found that a small but select squad of genes, involved in the development of nerve cells, are activated much later in our brains than in those of other primates.
This genetic delay mirrors other physical shifts in timing that separate humans from other apes. Chimpanzees, for example, become sexually mature by the age of 8 or 9; we take five more years to reach the same point of development.
These delays are signs of an evolutionary process called “neoteny“, where a species’ growth slows down to the point where adults retain many of the features previously seen juveniles. You can see neoteny at work in some domestic dog breeds, which are remarkably similar to baby wolves, or the axolotl salamander, which keeps the gills of a larva even as it becomes a sexually mature adult. And some scientists, like the late Stephen Jay Gould, have suggested that neoteny has played a major role in human evolution too.
As adults, we share many of the physical features of immature chimps. Our bone structures, including flat faces and small jaws, are similar to those of juvenile chimps, as is our patchy distribution of hair. A slower rate of development may even have shaped our vaunted intelligence, by stretching out the time when we are most receptive to new skills and knowledge. Somel’s research supports this idea by showing that since our evolutionary split from chimpanzees, the activation of some important brain genes has been delayed to the very start of adolescence.