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Evolution New data explains the difference between humans and chimps despite 99% DNA similarities
Published on April 02, 2006
In
February the world celebrated the Darwin Day to honor Charles
Darwin who proposed a scientific theory of evolution about 146
years ago. Darwin himself was not aware of the chemical nature
of gene. In late nineteenth century, geneticists thought that
genes are bead-like structure embedded in chromatin, which made
up chromosome. However, the lower organisms such as bacteria,
mycoplasma, etc., do not have any chromosome. In early
twentieth century, biochemists learned that nucleic acid is the
building block of chromosome. Later, four nucleotides were
identified, which made up the DNA. In 1950s, James Watson, a
Midwesterner from America, and Francis Crick, an English man,
deduced the three-dimensional structure of DNA with the help of
X-ray data and model building; Chargaff's Law that involves a
particular regularity involving the nitrogenous bases in DNA
(A=T and C=G) also helped the duo scientists. This seminal
discovery opened a floodgate of research in biology ushering a
new field called Molecular Biology.
In 1960s, the genetic code was cracked and t-RNA�s role in protein synthesis was elucidated. Three U.S. scientists received Nobel Prize in Medicine and Physiology (one of them, Dr. Robert Holley did the work in a building where I am located now at the U.S. Plant, Soil, and Nutrition Lab inside the Cornell campus). The enzymes for DNA manipulations were discovered throughout the 1970s leading up to the methods for gene splicing in late 1970s and ushering in recombinant DNA technology. A natural consequence of this revolution was DNA sequencing at a blazing speed, which spawned many new areas in life science, namely, genomics, proteomics, bioinformatics, etc.
The scientific world started to sequence the entire genome of some select bacteria, yeast, fungi, plants, and animals including mouse, chimp, and human. Because of the rapid progress in genomics, new insightful information had enriched the field of evolution. Charles Darwin had no idea what constituted a gene. He was a naturalist and field biologist who studied variations in beaks amongst finches without even knowing which genes are involved in beak formation in birds. Now, by comparing DNA sequences a scientist could draw inference about genes required for the development of a tissue or more precisely for coding proteins. Many of the proteins serve as enzymes (bio-catalyst), which are essentially nano-machine.
In the last decade, scientists have compared the DNA sequence of many mammals to look at the sequence similarities among different species and genus. One surprising discovery was that mouse and man�s DNA showed a tremendous amount of similarity (over 94-95% similarity). In the same vein, chimps and humans showed about 99% sequence or gene similarities. Therefore, scientists were puzzled and they ask � how come chimps and humans are so different but share 99% of the genes?
The answer came today (March 8, 2007). Thanks to the work of scientists from America and Australia who now say that changes in the gene expression and not the genes, is the reason humans are separated from their nearest relatives. A Reuters writer by the name Patricia Reaney penned a news article based on a recent publication in premiere scientific journal Nature.
Gene expression is a molecular process by which genes are turned on or off. Humans have of about 30,000 genes encoded in the chromosome but not all are activated or tuned on at the same time in every cell.
Prof. Kevin White, a professor in the department of genetics, ecology, and evolution at Yale University, who is a co-author of this recent Science article said, �We think gene expression is a major part of what separates chimps and humans.�
Prof. White collaborated with researchers from the University of Chicago in Illinois and the Hall Institute in Parkville, Victoria in Australia; they looked at gene expression in humans, chimpanzees, orangutans, and rhesus monkeys by using a newly developed scientific method call gene-array technology to compare the level of expression of 1,056 genes in the four species.
Prof. Gilad of University of Chicago who led the work said, �When we looked at gene expression, we found fairly small changes in 65 million years of macaque, orangutan, and chimpanzee evolution.� However, they noticed that mutations (changes) were more frequent in a specific group of genes known as �transcription factors� that control the expression of other genes. The researchers argued that since humans diverged from their ape ancestors during the last 5 million years, these changes took place in that span of time. The scientists say that this rapid evolution in transcription factors took place only in humans.
About thirty years ago, two researchers, Mary-Claire King and Allan Wilson, postulated that the key difference between humans and chimps might be in gene expression.
The research published in the journal Nature, supports a 30-year-old hypothesis by scientists Mary-Claire King and Allan Wilson who suggested that key differences between humans and chimpanzees might be in the way they express their genes. The hypothesis could not be tested in the 1970s due to the lack of technology. Now that genome mapping and gene array technology are available, evolutionists could test the validity of the hypothesis. Prof. Gilad and co-workers used samples of liver from five adult males from each of the four species (human, chimpanzees, orangutans, and rhesus monkeys) in their landmark study.
They found about 60 percent of the genes in the liver had consistent levels of expression in humans and the primates. However, genes for transcription factors were more likely to have changed their expression patterns than the genes they regulate.
To sum up the work Prof. White said, �Specifically in the human lineage the transcription factors are changing or evolving in their expression at a faster pace than in the other lineages, particularly as compared with chimps.�
The researchers are not sure what caused the shift in gene expression in humans but they think it could be due to certain changes in the environment. The research team plan to use other types of tissue to look at large arrays of genes in their future studies.
In summary, modern molecular biology tools have allowed researchers to figure out which sets of genes are changing faster than rest of the genes during evolution of humans from chimps. The answer came; it is the transcription factors that regulate other genes. Those other genes have not mutated at all. This explains why chimps and humans have nearly identical genes (99% similarities). However, because humans� transcription factors have mutated, they allow for different sets of genes to be expressed. That is the key difference. In humans, the genes that code for human-like features are expressed. Therefore, gene regulation at transcription level allowed by news sets of transcription factors is the key to evolution. This finding explained how humans look so different from other primates despite an astounding 99% similarity in DNA sequences. If Darwin could be alive now, he would be more than happier to know that progress in molecular biology is confirming his scientific theory of evolution. As per science, human and chimps parted their way millions of years ago; however, established major religions tell us that humans evolved only 5-6 thousand years ago. Which theory do you trust? Related Article:
Dr. A.H. Jaffor Ullah, a researcher and columnist, writes from Ithaca, New York.
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