"While Darwin's theory has explained the 'why' of evolution, much of the 'how' remains to be learned," Chuong adds. "Evo-Devo research promises a new level of understanding."
Why is a motive, typically unfalsifiable.
Scientists from the Keck School of Medicine of USC for the first time have shown experimentally the steps in the origin and development of feathers, using the techniques of molecular biology.
Their findings will have implications for the study of the morphogenesis of various epithelial organs – from hairs to lung tissue to mammary glands – and is already shedding light on the controversy over the evolution of dinosaur scales into avian feathers.
"The Morphogenesis of Feathers," a paper describing this work by principal investigator and Keck School pathology professor Cheng-Ming Chuong and his colleagues, was selected for online publication in the journal Nature.
"The feather is one of the best research models you can find for understanding the basic molecular pathways used by all epithelial cells," said Chuong.
"Scientists agree that whether you're looking at a human mammary gland or a chicken feather, epithelial cells use the same underlying logic, the same grammar, to form an organ. But unlike a gland, a feather really lays everything right out there for you."
The question of what makes a feather a feather has become rather heated in the recent past, with the discovery in China in the 1990s of fossilized dinosaurs like the Sinorthosaurus (Chinese-bird-dinosaur), with branching skin appendages on its skin.
"Some say these things are feathers, some say they're protofeathers, others say they're not feathers at all," Chuong said. "Everybody wants to know which one is the real first feather."
And they also want to know how it came to be. Over the years, Chuong said, paleontologists trying to trace the evolutionary connection between dinosaurs and birds have looked at the ways in which a reptilian scale might turn into an avian feather.
Most adult feathers have a backbone, or stem, called a rachis, off of which the feather's barbs branch. Each individual barb then branches again into the feather's smallest unit, the barbule, which is made of a single row of epithelial cells.
Downy feathers, like those on a chick, lack a rachis altogether and are made up of barbs studded with barbules.
The standing hypothesis among many paleontologists has long been that the scales on dinosaurs must have lengthened into rachides that then became notched to form barbs and barbules.
But there has been no real molecular evidence to either back up or refute that argument until now.
In their Nature paper, Chuong and his colleagues have demonstrated how barbs and rachides are formed in a modern chicken. They also have shown that the evolution from scale to feather most likely followed a path in which the barbs form first and then fuse to produce a rachis – rather than a rachis forming first and then being sculpted into barbs and barbules.
This interaction between evolutionary biology and developmental biology (dubbed "Evo-Devo") is a relatively new marriage of two previously disparate fields.
To reach their conclusions in Chuong’s laboratory, Mingke Yu, the postdoctoral fellow and first author on the paper, along with colleagues Ping Wu and Randall B. Widelitz, developed a novel way to genetically manipulate different genes during feather formation.
They plucked feathers from chickens, then prompted the chicken to regenerate those feathers under controlled conditions, raising and lowering the expression levels of the genes in question on an individual basis and observing the effects they had on the organization of epithelial cells into different feather forms.
Among others, three genes in particular – noggin, bone morphogenetic protein 4 (BMP4) and the whimsically named sonic hedgehog (Shh) – were found to result in new feathers that were rife with abnormal organization in their rachides and barbs.
When Chuong's team increased the expression of noggin, it found that the rachis began to split into several small, thin rachides, and the barbs increased in number.
When the group increased the expression of BMP4, with which noggin interacts antagonistically, it found that the feather's rachis became gigantic and its barbs merged and were reduced in numbers. In this way, the team was able to essentially manipulate the number and size of the feather's barbs and rachides.
And when they suppressed Shh, they found a residual webby membrane between the normally separated barbs.
"The cells there were supposed to go through apoptosis, or cell death, in order to create the space between the barbs," said Chuong. "But when we took away the sonic hedgehog signal, cell death no longer occurred. It is a similar process to that which occurs in the web of duck feet."
What can these new findings on the morphogenesis of feathers tell us about their evolution?
"These results suggest that the barbs form first and later fuse to form a rachis, much like downy feathers are formed before flight feathers when a chicken grows up. Downy feathers made only of barbs probably appeared before the evolution of feathers with rachides capable of flight," Chuong said.
"However, pinning down the exact moment at which dinosaur scales become chicken feathers is unrealistic. Just like Rome, feathers are not made in one process. It took 50 million years for nature to refine the process, to transform a scale into a flight machine. There were many, many intermediate stages," he explained.
" Darwin's theory has explained the 'why' of evolution, but much of the 'how' remains to be learned. Evo-Devo research promises a new level of understanding," Chuong said.
These findings also have medical applications, he added. "With this study, we learned more about how nature guides epithelial stem cells to form different organs. For example, BMP, Shh and noggin are also used in different ways in making lungs, limbs and spinal cords.
"By analyzing these models, scientists may be able to fully understand nature's 'grammar' and learn to use it in repairing or regenerating tissues and organs, which we call tissue engineering," Chuong said.
The research was supported by grants from the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health, and from the National Science Foundation.
Why is a motive, typically unfalsifiable.
Yes, an the evolutionists shamelessly assert such claims to be scientific fact - without even having looked at the facts! Without the 'how' evolution is plain nonsense. There is much to be said against the 'how' of evolution in numerous grounds.
Let me mention one problem with the feathers which is certainly very real. A mutation occurs in a single individual and for it to spread it must find mates willing to procreate with it. On one side evolutionists say that birds will not procreate with those which have a different feather coloring. Here we have a dinosaur with feathers - the first one! Now, not being a dinosaur, I cannot assert this with certainty, but it seems to me that the 'ugly duckling' syndrome applies here and that the feathers would make this dinosaur a very unlikely mate for other dinosaurs. This is a problem with many other new traits and functions posited by evoluton and it is a contradiction of the theory. Darwin spent long chapters in the Descent of Man on sexual selection saying how important it is. Such a radical difference, whether eventually favorable or not, would be considered a deformity as far as sexual selection is concerned and be unselected for.