Genetics Tell An ‘Out Of Africa’ Story
This year is the 50th anniversary of Darwin's book as well as the seventh anniversary since the completion of the human genome.
How did Charles Darwin buck the accepted theory of creation during the 1800s without the help of genetics? He was the ultimate zoologist, observer and collector.
Darwin had experts analyze hundreds of fossils and thousands of animals collected during his five-year trip to South America and the Galapagos Islands aboard the Beagle (1831-36). The ship was commissioned to map the harbors and coastline of South America. Darwin spent 30 years working on his theory before publishing his revolutionary book “The Origin of the Species.” His theory challenged the world to think of evolution by natural selection over the millennia. It was widely debated before being accepted during his lifetime.
This is the 150th anniversary of Darwin’s book, and this year also marks the seventh anniversary since the human genome was completed. The cat, dog and horse genomes are now also completed.
These achievements allow genomic scientists to track the true “origin of the species” by probing for shared ancestral mitochondrial DNA found in ancient bones from archaeological sites around the world.
The Molecular Clock
It is hard to believe, as we look at today’s genetic diversity, that humans, house cats, dogs and some horses shared in the migration out of Africa to the Middle East and beyond. The real story of the common ancestry of today’s humans and domestic animals predates history. With gene probes, scientists are solving many of these prehistoric mysteries. Scientists call this slow rate of genetic change the “ticking of the molecular clock,” while Darwin called it natural selection.
We now know that genetic diversity is due to different rates of genetic expression of only a small percentage of the body’s total DNA. The 3 billion DNA bases or “letters” in the genome of humans actually have less than 1 percent difference in the DNA makeup from chimps and just a bit more difference from other animals!
How could a few genes and their DNA sequences change the code over the millennia to evolve up to primates and then 6 million years ago to diverge chimps from man?
Why is there such a profound difference between chimps and humans? The 1 percent of unique pieces of DNA, made of 15 million bases or “letters” in our genome, makes us human. These genes are up-regulated or down-regulated to express traits that distinguish humans from chimps and other species.
Numerous regulatory sequences that do not encode protein or RNA are responsible for signaling genes to turn on or off at various times. Gene expression yields the various characteristics we know as human—our big brains, our prehensile thumbs and our speech! Natural selection or deliberate selection of certain traits or features that improved survival in the environment, or were considered desirable, further created genetic diversity manifesting today as various races and breeds.
The Dawn of Man
It is now accepted scientifically because of genetic mapping that modern humans did not originate from the Neanderthals. They are a separate species. Neanderthals dominated Eurasia for 200,000 years and died out 30,000 to 45,000 years ago without mixing with modern humans, who migrated from the African continent about 60,000 years ago.
Modern humans eventually differentiated into races, with numerous languages, customs, cultures, beliefs and politics as they populated the planet as we know it today.
Using genetic probes, scientists have identified the ancestors of domestic cats. House cats date to the Fertile Crescent originating from the Near Eastern wildcat population, which was domesticated by local farmers. The Near Eastern wildcat still roams the deserts of Israel, Saudi Arabia and other Middle Eastern countries.
Between 70,000 and 100,000 years ago the Near Eastern wildcat provided the genetic lineage that produced all domesticated cats. Five matriarchal lineages created the world’s 600 million domestic cats. Genomic studies suggest that ancestral continental migrations helped evolve 16 natural foundation breeds. Modern breeds were created in the past 50 years to total 50 recognized breeds.
Genetic analysis has found that dogs were first domesticated from descendants of wolves about 100,000 years ago in East Asia. Canine genomic probes have linked African and Asian breeds to a common ancestor that correlates with the migration patterns of various peoples from Asia into the New World.
Genomics also proves that our domestic dogs are not descendants of North American wolves. Genetic probes also revealed that ancient breeds—such as the Ibizan hound, the pharaoh hound and the Norwegian elkhound—are not included in the wolf-like ancestral group while the Siberian husky, the Afghan hound, Africa’s Basenji, China’s Chow Chow, Japan’s Akita, and Egypt’s Saluki are most closely related.
Most of today’s dog breeds have European origins: the mastiff-like breeds, including the bulldog, Rottweiler and boxer; the herding-type dogs, such as the Irish wolfhound, the collie, the greyhound and the Saint Bernard; and the scent hounds, which include terriers, spaniels and retrievers.
The Great Migration
Genetic probes of ancient bones show that domesticated dogs traveled with nomadic hunter-gatherers as they migrated across the Bering Strait from Asia into North America 12,000 to 14,000 years ago.
The diversity found in ancient New World dogs indicates that multiple lineages of dogs crossed the Bering Strait but did not pass their genes to modern dogs. Dogs are the only domesticated animal with New and Old World distribution before the arrival of Columbus to North America.
There are about 400 dog breeds in the modern world, with certain breeds having unique predispositions to genetic disorders.
It seems that ancient horses were domesticated in various regions from multiple genetic wild populations of lower genetic diversity. This gave rise to multiple genetic founder phenotypes from prehistory such as ponies and warm blood-like horses. Horse domestication may have occurred as early as 4500 BC in Iberia and the Eurasian steppe.
However, the first evidence was the burial of chariots dating 2000 BC in the Ural steppe. This was 800 years before chariots became commonly used from Greece to China. Horse power enabled conquerors and travelers and greatly influenced world history.
Studies of mitochondrial DNA have identified 17 geographically distinct clusters such as the Arab horse from Egypt. The unique D1 cluster includes Iberian (Andalusian and Lusitano) and North African horses (Barbs). The Spanish brought their horses to North America and their distinct DNA is found in 31 percent of American mustangs.
Studies found that the many equine breeds we know today derive their genetic diversity from 77 captured and domesticated mares that originated from various geographic herds of ancient wild horses. This makes equine genealogy mapping very complex.
A Greater Understanding
Knowing the genetic origins of various species certainly validates the One Medicine theory. Humans share vast genetic blueprints, including numerous inherited disorders, with animals. This commonality provides the rationale for studying genetic diseases in animal models to benefit human health.
There are about 200 genetically inherited disorders in cats, more than 300 in dogs and 25 or so in horses. Dogs share an estimated 150 of their genetic diseases with humans, and some breeds are prone to specific genetic disorders. Studying a prone breed helps researchers locate disease genes more efficiently.
Purebred dogs and cats manifest certain genetic disorders, which may be minimized or avoided with breed management plans using the genetic information available.
The Golden Retriever Club of America donated $500,000 to Morris Animal Foundation’s Cure Canine Cancer Campaign—CureCanineCancer.org—to fund genetics research into cancer genes.
Equine genetics information is not always being used as readily. This is because some performance qualities that win events, money and prestige are at times associated with an increased risk for certain genetic disorders. Pressure to produce elite winners are formidable motives on one side of the dilemma confounding breeders.
This is an issue for breeds with a high percentage of carriers of genetic disease, such as polysaccharide storage myopathy, which affects quarter horses, with 28 percent of cutters being identified as carriers.
Modern technology has created huge ethical dilemmas for society. One is the increase in gendercide. Asian and Indian cultures prefer sons, and this preference has migrated to almost every continent affecting rich and poor, educated and illiterate, including Hindu, Muslim, Confucian and Christian populations.
Selection in favor of boys is a combination of ancient prejudice and modern pressure for small families. It is estimated that 100 million baby girls have been aborted, killed or neglected to death, haunting mothers and perverting the family unit.
Now that families can afford prenatal ultrasound scans and private abortions, gendercide is increasing in the richer modern parts of China and India. This unnatural selection has skewed the sex ratio of Asian and North Indian societies.
The excess millions of frustrated single men forecasts a rise in crime, gangs, smuggling and violence. Is this too much information?
This article first appeared in the June 2010 issue of Veterinary Practice News. Click here to become a subscriber.
Alice Villalobos is a past president of the American Assn. of Human-Animal Bond Veterinarians and is president-elect of the Society for Veterinary Medical Ethics.