Darrell Sapp, Post-Gazette Lee M. Silver, professor of molecular biology and public policy at Princeton University, addresses a conference at Children's Hospital last week. Click photo for larger image.

More than 99.9 percent of all species that ever lived on Earth are now extinct. Who is Homo sapiens to break that mold?

There are but four ways the story of humankind can play out, says Dr. Lee Silver, a professor of molecular biology and public policy at Princeton University's Woodrow Wilson School of Public and International Affairs.

Least likely -- in fact it's practically impossible -- we might remain in more or less our present form for several billion years, awaiting impending doom at the hands of the dying sun. Alternatively, we might go the way of the vast majority of our ancestors, becoming archaeological fodder for the ages without having changed and to little ado. Probability prefers this bleak outcome.

Dr. Silver's other two scenarios involve changes in the human genetic code. We might be able to adapt as a single, interbreeding population. Future generations would be progressively modified, perhaps becoming taller, more fit and more intelligent, but remain Homo sapiens. The last option is also the most contentious: replacement by way of evolution. Our gene pool might give rise to a more worthy creature in the Darwinian sense, which would systematically replace us as ruler of Earth.

Dr. Silver laid his ideas before an auditorium full of medical students, physicians and bioethicists at the Children's Hospital of Pittsburgh Thursday. He said it was a scientifically founded foray into the realm of science fiction: not what will happen, what can happen.

"I think the real take-home message of Dr. Silver's lecture was that the sky's the limit, anything's possible," said Professor Alan Meisel, who teaches law and bioethics at the University of Pittsburgh. "We can do through scientific advances the same things that natural selection would do, only faster. We're probably capable of doing all kinds of things we can't even imagine today. ... We really have to suspend our imagination in order to determine our true limits."

Dr. Silver's hypothesis for the evolution of a successor to Homo sapiens, which he posits in his book "Remaking Eden," deals with the possible forms of "post-human" life and how society and humans might cope. Not surprisingly, the fledgling fields of genetics and biotechnology serve as the primary catalysts.

The cost of reading genetic coding has fallen exponentially since the landmark Human Genome Project began in 1990. A task once thought impossible, the federally funded effort to determine the sequencing of approximately 3 billion chemical pairs that compose human DNA succeeded in 2003.

"It's a matter of time before the analysis of individuals' genetic codes will become available on a cost-effective basis," said Dr. Silver, adding this will be the watershed moment that will potentially give humans power over their lineages that is unprecedented in natural history.

What's more, it's already possible to customize genetic makeup. Mice embryos have been infused with human chromosomes to make them better test subjects. Dolly, the cloned sheep that Dr. Silver calls "the most famous sentient being I've ever met," is perhaps the most memorable example of science's current capabilities, which can only grow in the future.

Customized DNA?

But what if prior to conception parents could customize their children's DNA based not on medicine, but preference? That's where things get tricky. Dr. Silver claims to have introduced this concept in 1997 with "Remaking Eden."

He calls it "reprogenetics": the confluence of reproductive selection and the yet-to-be-seen use -- or misuse -- of genetic technology therein. Not only would they be able to weed out disabling genetic diseases, they could also make their child stronger, more intelligent, or more beautiful.

Dr. Silver speculates that the technology would fall unevenly upon economic classes if brought to fruition. Monopolized at the hands of the few, the many that continue to reproduce naturally would be pushed to the peripherals of power.

Over the course of many generations, these social cleavages could become so divergent that speciation, or a split in our branch of the evolutionary tree, would occur.

Then their offspring would be unable to interbreed. If they mated, the children they bore would be sterile, much like mules are as offspring of a donkey and a horse.

But in order for the new species to totally replace humans, they would either have to defeat us by resource competition or engage in proactively sterilizing and killing the remaining population, culminating in the extinction of humanity as we know it.

We would not be the first hominids to be ushered out of existence in this manner. Recent archaeological studies have suggested that our ancestors, the Cro-Magnons, might have cannibalized Neanderthal man as they spread across Europe from their southern homelands.

Nor is history without instances of systematic murder and sterilization inflicted upon whole peoples. The term for the manipulation of demographics by controlling breeding is eugenics, of which the Nazis are history's most infamous proponents.

When can our kind expect this most callous of betrayals? Either in many thousands of years or never, says Dr. Silver.

He is not the only scientist to dabble in the many unknowns of the biological future.

Dr. Oliver Curry of the London School of Economics and Political Science was commissioned by the UK's Bravo network in 2006 to create a think piece on the possible futures of humankind. In that work, the Bravo Evolution Report, Dr. Curry speculated that the next 100,000 years could hold some unfortunate turns for our species.

'Celebrity' gene pool

Over a huge amount of time, writes Dr. Curry in the report, reproductive selection could "create more and more genetic inequality, and with it social and economic inequality -- so much so that the circles in which the genetic elites move become ever more exclusive, until they lose contact altogether with the rest of society, and come to constitute their own 'celebrity' gene pool.

At this point we may begin to see a parting of the ways between the genetic 'haves' and the genetic 'have-nots.' "

This line of thinking leads Dr. Curry to envisage two separate species: one "gracile" but enfeebled, much like the old European aristocracy, through an over-reliance on technology; the other "robust" because they remained subjects of nature. Dr. Curry cites H.G. Wells' Eloi and Morlocks from "The Time Machine" to illustrate his point.

But Dr. Silver and Dr. Curry seem to disagree on the effect of growing interconnectivity within the human population.

Humanity, recently reunited by advances in modern transport, is host to ever-growing genetic diversity. The more possible combinations, the greater variety will be produced.

But, said Dr. Silver, it's much smaller populations that are subject to the mutations that occur within their individual constituents.

Dr. Curry wrote that in a diverse population the most extreme individuals would rise to the top or sink to the bottom of the gene pool, in fact creating a greater chance for speciation to occur.

Dr. Silver sees little or no room for Darwinian natural selection to affect the human race as things stand.

The influence of societal institutions such as medicine and human rights are too great, the population too intertwined to allow for a single particular mutation to gain predominance over the whole.

In order for speciation to occur naturally, said Dr. Silver, there would have to be a significant bottleneck in human population.

Recent genetic studies have shown that all humans alive today descend from but a very few survivors of some catastrophe that occurred 70,000 to 80,000 years ago -- perhaps the eruption of a supervolcano that lay beneath the Indonesian island of Sumatra, which blew some 670 cubic miles of that island into the atmosphere about 74,000 years ago.

But a situation like that occurring in the near future is highly unlikely, he said.

"You have to remember that if 95 percent of our current population is killed off, there would still be more people living on the planet than at the time of Christ," said Dr. Silver.

He said that Severe Acute Respiratory Syndrome, or SARS, which is highly contagious and equally fatal, theoretically had the potential to produce just such a bottleneck during the outbreak several years ago had it not been stopped by a swift and efficient response from modern medicine.

Both Dr. Silver and Dr. Curry are quick to point out that these are only theoretical exercises, albeit ones founded upon real scientific truths.

Within our lifetime, the future of Man will remain the conjecture of brilliant minds. But as long as scientific civilization exists, its citizens must remain vigilant, said Professor Miesel.

"The question is whether we should allow [the use of biotechnology] to go any further. It's not just about the science, it's about how far the science can and ought to go," said the bioethicist.

"It's important that we as a society participate in making these decisions, which are not scientists' alone to make."