Nothing gets the blood boiling like a manimal. For many people, the idea of breaching the human species barrier--to mingle our biology with that of an animal--seems like a supreme affront to the moral order. In his January state of the union address, President Bush called for a ban on "creating human-animal hybrids."
These so-called chimeras, according to their opponents, devalue humanity by breaching our species barrier. "Human life is a gift from our creator, and that gift should never be discarded, devalued or put up for sale," Bush declared. Senator Sam Brownback of Kansas expanded on this sentiment in his Human Chimera Prohibition Bill of 2005. Chimeras, according to the bill, "blur the lines between human and animal." They must be banned because "respect for human dignity and the integrity of the human species may be threatened by chimeras."
Some opponents cite the Bible as proof that chimeras are wrong--in particular, I Corinthians 15:39: "All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds." Others rely on their own sense of disgust as a reliable guide to the wrongness of chimeras. "When we start to blend the edges of things, we're uneasy," explains Grant Hurlburt, a psychiatrist and member of the President's Council on Bioethics. "That's why chimeric creatures are monsters in mythology in the first place."
So let's imagine, for the sake of argument, that a nefarious plot to create human-ape hybrids was discovered in some distant country. Genetic studies revealed at some point in the past, the DNA of that nation's citizens had been mixed with the DNA from a separate species of ape. The species barrier had been breached, and the contamination had been carried down from generation to generation. Even today a significant amount of DNA in people of that nation could be traced to that hybridization. Horror and condemnation would arise at the discovery that chimeras had been created.
But of course, this is all hypothetical, right?
Scientists know a fair amount about how species split apart--including the genetic changes that occur during the split. An old species splits into new ones when one of its populations becomes isolated. The individuals in a population begin to interbreed more with one another than with the rest of the species. When mutations emerge in that population, they don't spread to the rest of the species.
Over time, the population becomes more and more genetically distinct from other members of the species. They may begin to look different, and they may begin to specialize in different ways of getting food. They may still be able to interbreed with other members of their species, at least in theory. But barriers may emerge. A population of plants may begin to flower at a different time of year, for example. Fish may prefer to mate with fish that look more like themselves.
If the population has some contact with the rest of its species, a few hybrids may be produced in each generation. In some cases, they more form a healthy hybrid zone. In other cases, the hybrids may not thrive as well as their parents. Nevertheless, through this interbreeding, genes may flow back and forth over the barrier--even as the barrier is becoming stronger. It may take thousands or millions of years for a new species to bud off completely from an old one.
Most of these insights have emerged from studies on living species--observations on the mating habits of fruit flies, measurements of genetic divergence in wolves, and the like. (See the 2004 book Speciation if you crave the full story.) It's also possible to use these insights to learn about the origin of species in the distant past. What you need is a ton of data--such as genome sequences--from living species, and fossils to provide points of comparison. And thanks to the human genome project--as well as the chimpanzee genome project, and similar efforts to sequence DNA in gorillas, orangutans and other apes--the origin of our own species has become one of the best cases to study. In tomorrow's issue of Nature, a team of scientists from the Broad Institute in Massachusetts present the biggest such study by far.
Before this study, the rough consensus among scientists was that our ancestors diverged from the ancestors of our closest relatives--chimpanzees and bonobos--at some point between five to seven million years ago. That evidence came from studies on DNA, as well as from fossils, such as the oldest hominid fossil, Sahelanthropus, which is estimated to have lived 6.5 to 7.5 million years old. Most scientists argued that the hominids made a relatively clean break from other apes, without any significant hybridizing.
But that's not what has emerged from the new study. The Broad Institute scientists lined up millions of bases of DNA in humans and chimps and measured their differences. Humans and chimpanzees both inherited each segment of DNA from a common ancestor. Over time, the copies of that ancestral segment picked up mutations. The differences between them can offer clues to how long they've been evolving along separate paths. It turns out that the ancestors for some of those segments are much older than others. The only way to make sense of these results, according to the scientists, is to conclude that hominids and the ancestors of chimpanzees were interbreeding--to some extent at least--for four million years.
It's hard to get a fix from the new evidence on exactly when our lineage split from the chimp lineage, and when the interbreeding finally stopped. But the authors do question whether Sahelanthropus and other early hominid fossils represent the start of a distinct line of hominids that remained pure up to living humans. They suggest that the two lineages did split apart, and hominids began to evolve the distinct hominid body (walking upright, perhaps). But later the two populations interbred, mixing their genes. So our genes have a younger ancestry than the fossil record of hominids.
Some of the most intriguing signs of hybridizing comes from the X chromosome, one of the sex determining chromosomes (XX is female, XY is male). The X chromosome contains segments of DNA that have the most recent common ancestry of all the DNA shared by chimps and humans. The scientists suggest that when the ancestors of chimps and humans came back in contact a few million years ago, hybrid males turned out to be infertile--a common pattern in the origin of species among many animals. Only the females remained fertile, returning to their populations to mate and spread the genes from the other species among their own. If they had sons, those males would be fertile. As a result, many old copies of genes on the X chromosomes were lost, and the X chromosomes in today's chimps and humans share a recent ancestry.
Smaller previous studies have suggested hybridization, but they've been rejected by a lot of other experts. It will be interesting to see how this one fares with the critics. I suspect it will be much harder to contest, because the sample is so vast--800 times more DNA than in previous studies. If it holds up, it raises all sorts of fascinating scientific questions. What brought the two lineages back in contact? Were they fleeing some ecological disaster? How did the two populations interact? Why didn't the two lineages collapse back into one species? Did our ancestors evolve some mating preference that turned them away for good? And how did an infusion of chimpanzee-ancestor genes alter the workings of the bodies of our ancestors?
And then there is the matter of the manimals. I'm not trying to says hominids interbreeding with proto-chimpanzees are exactly the same thing as mice with human neurons growing in their heads. Obviously there's a biological difference here, because mice are separated from us by far more evolutionary time. I would just argue that any ethical stand on chimeras has to be in harmony with the scientific evidence. And it doesn't seem as if there's much careful thought behind the call for a total ban on chimeras. Integrity is not essential to a species. It emerges gradually over time, through evolutionary change. And when a new species starts to emerge, its integrity is not a foregone conclusion. Politicians may want to dismiss hybrids between animals or between plants as irrelevant to the issue of human-animal hybrids. Humans are different, the argument goes. It's certainly true that we are different from all other animals in our cognition and other faculties. And it's right to use those differences as a starting point for deciding what the ethics of new biotechnology. But when it comes to the integrity of the human species compared to other animals, there is no difference. That fact remains that our upright hominids apparently intebred with the ancestors of chimpanzees. We are all the children of chimeras.
Update 5 pm: John Hawks takes issue.
Superb post, you have a real talent for making this kind of thing easily understandable to non-specialists. Thank you.
Regretably this will couse a backlash amongst a certain (too large) segment of the public that don't even accept that humans and apes have a common ancestor, never mind that we may have continued to interbreed for thousands or millons of years.
Has there been enough DNA recovered from Neanderthalensis remains for a hybridization study like this to performed for Sapiens and Neanderthalensis?
I haven't read the article yet, but their interpretation seems to rest on the (uncontested) idea that all DNA sequences within a species mutate at the same rate. That's a very big assumption.
Where are bonobos in this? Did they split off from chimpanzees before or after the human-chimp split?
A commenter pointed out on my site that a paper from 3 years ago comes to pretty much the same conclusion (minus the dates) as this new paper. The Osada and Wu paper would be a better comparison to this new study than the Navarro and Barton paper that you link to. Navarro and Barton attempted to stretch a model for chromosomal speciation that works for Drosophila into a model to explain molecular evolution in primates. Other publications showed both their assumptions and analysis were incorrect.
There is some unpublished data suggesting hybridization between neanderthals and H. sapiens sapiens. The conclusion was reached without using any neandethal DNA -- it's a purely stastical analysis using coalescent simulations and human DNA sequence polymorphism. Look for it to cause a buzz when it gets published.
Would those who are repelled by human-nonhuman chimeras object to inserting human genes in yeast or bacteria to produce human insulin for example?
Has there been enough DNA recovered from Neanderthalensis remains for a hybridization study like this to performed for Sapiens and Neanderthalensis?
Oddly enough, Neanderthal gene sequencing has also been in recent news (it's probably a little too early to tell if it will be useful):
> Only the females remained fertile, returning to their populations to mate and spread the genes from the other species among their own. If they had sons, those males would be fertile. As a result, many old copies of genes on the X chromosomes were lost, and the X chromosomes in today's chimps and humans share a recent ancestry.
Ugh. So science now has good evidence that we Men really are the outdated brutes that Women accuse us to be? ;-)
Excellent summary, thanks.
An editorial note: More context in the feed please. I skipped this article based on the first line excerpt in your feed because there was no clue about the subject. I came back via Pharyngula, who told me what you where talking about.
After reading the original article and Hawks critique I'm of the opinion the too much is being made of this study regarding an initial divergence and subsequent hybridization of the chimpanzee and human lineages. There is also some question regarding the calibration point of 20 mya for the macaque (i.e. cercopithecoid/hominoid) divergence. This was more likely on the order of 25 mya based on inferences from the fossil record. Given an older calibration point the human/chimp divergence is pushed back to between 6-8 mya, more in keeping with the fossil record. I think the significance of this and similar studies however has been lost in the hubbub. What is demonstrated is that between 5-8 mya we're dealing with a population that probably occured before, during, and immediately after the last common ancestor of the African ape and human linages. For all intents and purposes we cannot differentiate the separate lineages at that time since they were very incipient and entirely capable of interbreeding. Sahelanthropus, Ororrin and Ardipithecus, purported "hominids" from between 5 and 7 mya should best be thought as broadly representative of the last common ancestor (LCA) of the African hominoids including us humans. When we begin to study this LCA we see that it may very well have had some features that were more human-like than ape-like in terms of its dentition and posture. This leads to the interesting possibility that the LCA was perhaps more similar in these respects to australopithecines than to modern apes. The human lineage may well preserve ancestral features of the LCA lost or altered in the gorilla and/or chimpanzee. That I think is the true significance of this and similar studies.
The facts are the facts and the DNA doesn't lie. The explanation posited by the paper is likely the simplest one explained by the data. There likely was little difference between proto-hominid and proto-chimpanzee until changes occurred in the X chromosome of what would be humans to prevent interbreeding with what would be chimps. Besides, we didn't possibly become fully human until maybe 1 million years ago and Homo sapiens nearly died out 100,000 years ago. Is there any evidence for such a population bottleneck in chimps that we haven't created from the bushmeat trade? The paper will live or die on its data and analysis until other evidence comes along to disprove it, however, it is arguing over the distant past and whatever happened back then was necessary to form an ape that today can ask such questions and try to find the answers to them. If we can't handle the truth derived from our own bodies and curiosity, then what does that say about us as a species?
"That's why chimeric creatures are monsters in mythology in the first place."
Centaurs or pegasus weren't considered as monsters.
Everytime I hear the word 'chimera' I keep thinking, a man with a horses head. But here is a very different perspective to chimeras
Like you said, it looks like we are all children of chimeras!!
Does this new research bolster the human evolution story or raise more doubts about it? Here's what the science reporter for the Harvard Medical School writes:
The surprising findings reveal a much more complicated birth of the human and chimpanzee species than shown by previous research. They also call into question the place on the primate family tree of fossils that scientists had thought were the bones of ancestral humans, but which are older than the newly determined time that the species diverged.
Because mutations in DNA occur at a steady rate, scientists are able to compare the changes between species and figure out how long ago they last had a common ancestor.
[[This is not true. The molecular clock is not constant:
What the results reveal is a surprisingly large range. Different segments of the genome differ in age by about 4 million years
"The data were very, very unexpected and difficult to explain by what we knew," said David Reich, assistant professor of genetics at Harvard Medical School.
"The genome analysis revealed big surprises, with major implications for human evolution," said Lander, who serves as Broad Institute director. "First, human-chimp speciation occurred more recently than previous estimates.
By pushing human-chimpanzee speciation up by a million years, the research raises questions about several prominent fossils, such as the Toumai fossil, Sahelanthropus tchadensis, which scientists have dated to between 6.5 million and 7.4 million years ago.
Maybe hybrids that successfully adapt occur only once every million years," Reich said, adding that if hybridization between human and chimp ancestors did occur, "Either we're the hybrids or the chimpanzees are the hybrids, but we can't tell which.
To NelC comment 4.
Very insightful comment.
From Wikipedia: http://en.wikipedia.org/wiki/Bonobos
The Bonobo has a graceful body. Its head is smaller than those of the Common Chimpanzee but with a higher forehead. It has a black face with pink lips, small ears, wide nostrils, and long hair on its head. Females have slightly prominent breasts in contrast to the flat breasts of other female apes, though not as prominent as those of humans. The Bonobo also have slim upper bodies, narrow shoulders, thin necks, and long legs compared to the Common Chimpanzee. These characteristics, and their posture, give Bonobos a more human-like appearance than that of Common Chimpanzees.
If I had been a bipedal hominid back then, self preservation would dictate that Bonobos Pan paniscus (promiscuous?) would be less risky (risque?) sexual partners than Common Chimps Pan troglodytes.
I think people are worried about the possibility of us creating a slave species of human-ape hybrids-a topic of science fiction speculation.I don't know whether this would be scientifically possible;e, though.
For too long, science has hung onto the old religious idea that humans are separate from animals.
The First Nations people had it right -- there is no such thing as people vs. everything else.
Everything is People.
It's a concept that has nothing to do with maintaining a rank hiearchy, but of recognizing intrinstic worthiness of respect for everything.
Some centaurs were okay in Greek mythology; others were murderous bands of rapists (I agree that chimeric creatures have not always been negative, mythologically).
But the Chimera from which we took the term chimera was definitely a monster, which is why we took that term instead of calling cross-family hybrids pegasuses or gryphons.
Oh, that (Human Chimera Prohibition Bill of 2005) is just too funny.
Congress finds that--
(1) advances in research and technology have made possible the creation of chimeras, which are beings with diverse human and non-human tissue;
Advances in research and technology suggest human-human chimerism is common, perhaps universal.
(2) serious ethical objections are raised to some types of chimeras because they blur the lines between human and animal, male and female, parent and child, and one individual and another individual;
Yes. Welcome to the wonderful wacky world of mammalian reproduction.
(3) respect for human dignity and the integrity of the human species may be threatened by chimeras;
Respect for the intelligence of congress-critters may be threatened by bills like this. Oh, wait. Never mind.
(4) the uniqueness of individual human beings is manifested in a particular way through their brain and their reproductive organs/cells; and
Just not in the overly simplistic way you remember from high-school biology.
(Finding 5, that human/non-human chimeras may be a transgenic contagion risk, is at least vaguely plausible).
Definitions(1) HUMAN CHIMERA- The term `human chimera' means--
(B) a human embryo that consists of cells derived from more than 1 human embryo, fetus, or born individual;
Repeat after me, "I am quite posibly a human chimera, and I'm proud".
Sec. 302. Prohibition on human chimeras
(a) In General- It shall be unlawful for any person to knowingly, in or otherwise affecting interstate commerce--
`(1) create or attempt to create a human chimera;
Congress outlaws sex, news at eleven.