19 JULY 2007
Evolution is complicated. That's why it took a genius -- namely, Charles Darwin -- to figure out how it happened. And that's also why so many people today have a hard time believing that it happened at all.
It's
a peculiar sort of reasoning that surfaces in many arenas where science
confronts "common sense." Quantum physics, the Big Bang, global warming,
cancer from second-hand smoke, whatever -- many people seem to think if
"common sense" (theirs) cannot conceive of something, it therefore must
not be true.
Same for evolution. Many people can't imagine that amoebas and worms and flies transformed themselves into rats and cats and humans. If scientists cannot clearly explain every step in the process, this reasoning goes, the idea of evolution is consequently false.
It might be an understandable reaction, but it is nevertheless nonsense. A lot of fantastic things are undeniably true, even though science cannot explain everything about them. Gravity itself is true but mysterious, and what went up still came down long before Newton and Einstein came along to explain it.
With evolution, Darwin's theory about natural selection does explain many aspects of the history of life. Just not everything, yet. But there is progress in explaining how ancestral annelids and insects became rats and cats and basketball players.
By its very nature, evolution preserves a record of its past in the DNA code
that provides the blueprint for life's important chemicals. In recent
years, some of those chemical clues have revealed a possible answer to
the riddle of the emergence of animals with backbones. 
Those animals, who prefer to be called vertebrates, differ from their primitive ancestors in numerous ways. Besides their bony internal skeletons, vertebrates have a more elaborate system of organs, more powerful and more versatile muscles, and intricate immune systems, capable of identifying and fighting off a diverse array of diseases.
Flies did not evolve to possess all these properties by getting trapped in a transporter tube with Jeff Goldblum, merging their DNA with his. Rather there was a merger on a more modest scale. Certain Machiavellian molecules conspired to create a new system of cellular communication, conferring biochemistry with vast new powers.
That new system started with calcium, the element you need to build strong bones. Calcium has many other jobs, particularly as a sender of signals. When calcium enters cells, it launches a chain of biochemical reactions impinging on functions ranging from muscle contraction to learning and memory.
Skipping the messy details, calcium's ultimate effect is to send chemicals into the cell's nucleus, where they attach to DNA, the molecule that genes are made of. What a cell does depends on which genes are "turned on" to produce proteins and other molecules that keep life going. By attaching to DNA, outsider molecules determine which genes are on or off, thereby controlling the chemical processes maintaining life.
All this was true back when worms and flies were the paragons of advanced civilization. But their repertoire of chemicals for managing gene activity was rather limited. Attaching to DNA requires a molecule that can get a grip, and with only a few such molecules around, primitive life's responses to chemical signals were simple.
But about 500 million years ago, a gene for a molecule sensitive to calcium merged with a gene for a molecule that could grip DNA. Calcium then became a prime mover of cell activity. Over the generations, the original new gene duplicated itself into four variants that produce NFAT proteins (for Nuclear Factor of Activated T cells, the cells in which they were first found).
NFAT proteins alone can grip DNA only weakly, and so require a partner molecule to regulate genes effectively. But the need for a partner is a bonus, as NFATs can enlist various partners for attaching to different DNA sites. That gives the NFAT-partner complexes the ability to control a wide array of genes. And so calcium, which activates the NFATs, is now a very versatile messenger, affecting a whole host of cellular functions, particularly those that involve communication and cooperation between cells. And that's just what a body needs to build bigger organs, better muscles, and more versatile defense systems.
This story has been pieced together from catalogs recently completed of the total genetic endowment -- the genome -- of 16 vertebrate species. The NFAT genes are found in all the vertebrates but not in invertebrates, supporting the suspicion that vertebrate evolution is connected with their arrival on the scene about 500 million years ago, as Stanford University biologists describe in the June issue of Trends in Cell Biology.
Furthermore, experiments disabling NFAT genes have been shown to disrupt the development of organs and other essential vertebrate features, providing further evidence that NFAT proteins play a key role in providing vertebrates their defining qualities.
"The development of many vertebrate-specific organs is, indeed, dependent upon NFAT signaling," Stanford's Gerald Crabtree and collaborators write.
In fact, NFAT signaling appears to be essential in the vertebrate immune system, in the nervous system, and in producing heart and skeletal muscles. NFATs also play a role in skeletal bone formation, the most obvious vertebrate feature.
No doubt other forces were at work when life first advanced beyond spinelessness. But NFATs show how complex life can arise from primitive precursors, and that fantastic ideas defying common sense do not have to be easy to understand to be right.
E-mail: tsiegfried@nasw.org
