RELATIVELY CLOSE?: Seven weeks after the publication of Ardi's description in Science, the debate continues about where the species belongs in the evolutionary tree.
Image: J. H. MATTERNES/AAAS
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For such a petite creature, the 1.2-meter-tall "Ardi" (Ardipithecus ramidus) has made big waves in the paleoanthropology world. The momentous find—announced 15 years ago and formally described in Science this October—has deepened academic debates about when bipedalism evolved, what our last common ancestor with chimpanzees looked like, and how some ancient primates gave way to modern humans.
"This is a fascinating fossil no matter what side you come down on," says William Jungers, a professor and chairman of the Department of Anatomical Sciences at the Stony Brook University Medical Center in Long, Island, N.Y. The 11-paper Science analysis has, indeed, sharpened more differences than it has smoothed over.
The authors of the papers, including Tim White of the University of California, Berkeley, propose that Ardipithecus was "an effective upright walker" and that it "resolves many uncertainties about early human evolution, including the nature of the last common ancestor." But many others in the field propose that some of these statements may be overblown. In fact, Jungers says, "I think some of the things they said might have been for effect."
So, does Ardi represent a true step toward humanity, or should she remain up in the side branches of the evolutionary tree? White and his fellow authors do not propose to have a definitive answer, but through painstaking analysis of the fossil data and surroundings, they conclude in the overview paper that, "There are no apparent features sufficiently unique to warrant the exclusion of Ar. ramidus as being ancestral to Australopithecus," thus proposing she might indeed be an early hominin (the ever-changing nomenclatural group that usually includes living humans and our close extinct relatives, also referred to by White et al. as hominids—although the latter title now often includes the great apes, as well)
But piecing together how Ardipithecus fits into the evolutionary story of humanity may prove even more difficult than reconstructing Ardi's fragmented and fragile bones, and the process has already turned out to be a contentious one.
Able biped?
Because the traditional hallmark of an early human has been the adaptation for upright walking, much of the debate over Ardipithecus's status hinges on how her lower body bones fit together—in particular, the position of her damaged ilium, the winglike upper pelvis bone. Depending on how this bone is oriented, muscles around the hip joints work differently, explains David Begun, a professor in the Department of Anthropology at the University of Toronto. The summary in one of the Science papers, led by Owen Lovejoy of Kent State University in Ohio, argues that by Ardi's time, "the gluteal muscles had been repositioned so that Ar. ramidus could walk upright without shifting its center of mass from side to side" (unlike today's lumbering great apes), but a different interpretation of the ilium could change all of that.
Despite the numerous images and descriptions put forth by the researchers, others are reluctant to take the reconstructions without a grain of salt. Begun says: "Maybe the pieces do fit together nicely, but the reality is they start out with a very damaged specimen, and they end up with something very similar to an australopithecine" (the group that includes "Lucy," the 3.2-million-year-old Australopithecus as well as a 2.7-million-year-old Paranthropus). "It's very difficult not to make them look like something you have in your mind if there's any chance of play," he says. Jungers also notes the perils of reconstruction, which in a case like Ardi's "requires a lot of guesswork."
As the upper pelvis appears like it could belong to an early human, the bottom part looks more like a quadrupedal, nonhuman primate, says Jungers, who recently met with White and examined photos of the bones. White asserts, however, that after working with the fossils himself, there is no way that they could belong to "an animal that wasn't often walking on its hind legs," unless the data "were deliberately ignored, or if we had made them up," he argues.
Comfortable climber
Even if Ardi's reconstructed hips don't convince everyone, her feet could provide some important insights into the species's locomotion. In a summary to one of the Science papers led by Lovejoy, the authors note that, "although the foot anatomy of Ar. ramidus shows that it was still climbing trees, on the ground it walked upright." Ardi's feet do point to a comfort with life in the trees. Her big toe, which Jungers calls "remarkably primitive," is quite divergent—even more so than the grasping digit in modern-day chimpanzees—which would help with climbing.
None of the known foot components, no matter how well adapted to climbing, preclude Ardipithecus from walking upright on the ground. Jungers, however, thinks "it really doesn't show any adaptations for bipedalism at all." In fact, he says, many components of Ar. ramidus don't make Ardi look that much more adept at walking upright than chimpanzees—a primate that White et al. disavow as a model for early human evolution. In a summary paper led by Lovejoy, the authors describe Ardipithecus as a "facultative upright walker," one that can walk on two legs if needed (to carry something in the forearms, for example) but that isn't necessarily prone to do so.
"What's ironic is that that's how you would describe bipedality in chimpanzees," Jungers says—"they're facultative bipeds." Homo erectus, on the other hand, which lived about 2.6 million years after Ardi, were obligate bipeds, and he points out, "even humans are facultative climbers."
No matter how some of Ardi's bones are assembled or reassembled, the debate about how the species got around on the ground may not be settled by further analysis of this specimen. Even though hundreds of bones were uncovered, the species still lacks a knee joint. "I think a knee joint would seal the deal one way or another," Begun says. And more foot bones wouldn't hurt either, Jungers notes.
Cranial clues
Rather than continue the lineage debate below the belt, Ardi's most important features might rest above her shoulders, Jungers says. "We have to abandon bipedality as the hallmark of being a 'hominin' sensu stricto if we hope to keep Ardi in our clade," he wrote in an e-mail to ScientificAmerican.com
"If we had just found the fossils below the neck, it's possible we wouldn't be talking about Ardi as a hominin at all," he says.
The numerous cranial pieces that the research team uncovered might, however, help sway the debate toward the early human camp. In a conversation with White, Jungers says, he was compelled by the dental evidence—especially the upper canine teeth, which were smaller and more humanlike than those of chimpanzees—to consider Ardi as an early step in human evolution. The small canines and minimal size difference between males and females of the species are "indicative of minimal social aggression," the authors of one of the Science summaries (led by White) wrote. If males didn't compete for females through physical aggression, co-author Lovejoy has argued, they might have been more involved in raising offspring—a key component of later human evolution.
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16 Comments
Add CommentHa, I always thought the most genetically similar animal to humans are Pigs, did anyone ever check out Arnold the Pig to see if he is related???
Reply | Report Abuse | Link to thisrlb2
Reply | Report Abuse | Link to thisWhere is your information from, CSI Los Vegas?
And yes, Arnold would have a relation to you, he's just much more distant than than a chimp.
Coolmoss wrote- Where is your information from, CSI Los Vegas?
Reply | Report Abuse | Link to thisAnd yes, Arnold would have a relation to you, he's just much more distant than than a chimp.
rlb2 reply - if we are going all the way back to the Ardipithecus, ARDI is a distant relative of Humans then what is next??? Remember my relatives are your relatives so have an ork, ork day -- that's "have a nice swine day"...
"The possibility of pig to human organ transplants took a leap forward on Christmas Day when a litter of genetically modified pigs was born in Virginia. Simon Jeffery explains
The animals are sometimes called "horizontal humans". Although they are more distantly related to us than, for example, the great apes - pigs are about the right size, and so are their organs. A 75kg pig has the same-sized heart as a 75kg human, with the same pumping capacity. In theory it should be possible to farm pigs for their organs, much as we now farm them for bacon."
"There are currently more than 5,500 people in Britain waiting for a transplant of a kidney, pancreas, heart, lungs, liver, or a combination of these organs. On average, adults have to wait more than 500 days for a new kidney. There is a clear demand. Efforts are being made to increase the donation of human organs - the supply is still not high enough, though some argue that pig to human transplants would be unnecessary if the taking of healthy organs from the dead was mandatory."
"There may be a degree of revulsion at killing an animal to save a human, but some could feel happier carrying the organ of a dead pig than a dead human. Pigs are already bred and killed for food, but some vegetarians and vegans might feel uneasy about making such use of an animal. People who follow Jewish and Muslim eating codes forbidding the consumption of pork may also have objections."
http://www.viewingspace.com/genetics_culture/pages_genetics_culture/gc_w01/piglets/guardian_piglets/guard_piglets.htm
Ork Ork, Ork..........
rlb2:
Reply | Report Abuse | Link to thisThe biggest threat to human existence is its population size. The best solution is to shut down all medical interventions. When your body fails you die - its natural. If you believe in a god, its his/her will and he/she is going to take you to a better place, my mind will not entertain this delusion, so its harder for me.
The current refugee problems from Africa and Asia are the result of conflicts - the direct consequence of overpopulation. Expect to see billions of refugees in the not too distant future.
As a child from the baby-boom generation I am now reaching the end of my life, don't intend to intervene to extend it. My generation will be the first generation whose lifespan will be shorter that its parents. Keep watching the graph of the median life expectancy of humans the trend is about to flatten and then decline.
PS: There is no threat to the planet, that never existed before. It will nullify the excesses of our species and return to its normal balanced life supporting existence till its time eventually runs out.
Such a Great Magazine!! So many pig-headed readers!
Reply | Report Abuse | Link to thisPeterT
The wrist bone that is more primate like is in most other animals indicitive of knuckle walking. This keeps the hand structure stiff. A chimpanzee hand can not bend in such a way that their hand will lie flat on the ground. What sort of evidence are they working with that says that knuckle walking is not something that Ardi did?
Reply | Report Abuse | Link to thisscientific earthling - The biggest threat to human existence is its population size.
Reply | Report Abuse | Link to thisWhat you said in the one line above is true human activity is the biggest threat, human tribal warfare is the second biggest threat, meteor impact is the third biggest threat, loosing ones since of humor it the fourth biggest threat...
If you will clear your mind of the political (and even esoteric) notion that all human evolution must occur in "Ethiopia", and look objectively at the early (really entire) African record, you will see new, radically more advanced hominoid types appearing from nowhere and cohabiting with more primitive forms for lengthy periods.
Reply | Report Abuse | Link to thisThat nowhere may quite well be somewhere called Eurasia. The tangled development of bipedalism in Africa could be due to a series of (semi-)bipedal apes from a third great ape branch, other than the orangutans and gorillas-chimpanzees, walking into Africa and breeding with the local hominoids.
Perhaps they are the source of human blood type B, so mysteriously missing from the alleged sole human ancestor the chimpanzee. Perhaps they are what is doing the 4 million years of "chimp/human interbreeding" in the Patterson et al. genetic study (Nature 441, 1103-1108 29 June 2006). Perhaps they are why our Ardipithecus is something of a chimera.
SWEET!!!!!!!!!!!!!!!!!!!!!!!!!
Reply | Report Abuse | Link to thisSWEET!!!!!!!!!!!!!!!
Reply | Report Abuse | Link to thisI have often wondered whether the evolution of the human species might have occurred several times. By that I mean that the same evolutionary steps were taken under completely different conditions, possibly hundreds of thousands of years apart, in different places but from the same common ancestral group. I'm thinking, for example, that one such evolutionary development might have occurred in, say, Africa, while others occurred in Southeast Asia, the Hindu Kush, modern Australia etc. Coming from the same closely-related primate group, wouldn't they have near-identical DNA? If that were so, wouldn't it explain the problem-free interbreeding but maybe account for the differences in blood group prevalences and those genetic diseases that are specific to certain racial groupings ?
Reply | Report Abuse | Link to thisSometimes it is very hard to make sense of detailed facts when one can not see the whole picture. It is even harder when all the details are already so numerous it has numbing effect to trillions of possible connection to billions of neurons in the brain.
Reply | Report Abuse | Link to thisIs Ardipethicus ramidus the common ancestor of the chimpanzees and us? My instinct is telling me we getting closer to the answer.
Difference in adaptation may have been amplified by difference in climate, environment , and situations. That may explained why a species will branch out to two different species, and maybe more.
The success in prolonge survival of an initial species may depend on the many relative capabilities it may progressively possessed to adapt and prosper.
This reminds of the case of Neanderthals. Why did their successfull species died down? A probable reason may be they become so specialize in the one or the other climate and environment of Europe. Their high calorie intake and meat eating requirements to survive really push them to fewer kind of capabilities. They often hunt for huge omnivorous animals. This is what they exclusively eat. So when the climate/environment in Europe swing to extreme ice cold to extreme drought so many times, it will not allow them to adapt/evolved, sothey slowly diminished and vanished
In Africa on the other hand had a megadrought but with a couple of spots wet enough to have forest and savannah. This wet spots were close by the sea. This climate/ environment surely broaden the Homo Sapiens capabilities to adapt.
So when the situation push the forebear of homo sapiens out of Africa, they were to become the successfull colonizer of the remaining world.
As for Ardi, I believe his bipedal and tree climbing capabilities may have made him a possible ancestor of both forebearer of Chimpanzee and Human.
Hello,
Reply | Report Abuse | Link to thisI greet all of you, and I respect all of your comments, as well as your opinions.
I have read alot about evolution, which seems to be developing everyday.
But I sometimes ask myself: What is the basis of evolution??
Ofcourse, we all know the fact that whan a cell proliferates or divides, and subsequently produces a different number of chromosomes is mutated, and will eventually die.
But, have looked at Darwin's theory of evolution, which is the transform of champanzees ( a 48 chromosome organism) to human ( a 46 human organism)?
And if this was true, where is the missing link between apes and humans??
Why are apes still there, and not all of them evoluted to humans?
Why dont we suggest in the first place the humans evoluted to champanzees???
One onswer will eventually jumb to all of our brains, which is:
( ORGANISMS FROM DIFFERENT SETS OF CHROMOSOMES CAN NEVER EVOLUTE TO EACH OTHER)
This is my hypothesis.
If you have any comments, please elaborate and I respect and welcome all of your opinions
Thanks for reading.
Regards,
Wayel
Wayel, there are quite a few problems with your post, the first of which is your claim to have read a lot about evolution. This is clearly untrue. For starters, you use the word ‘evolute’, which is a term used in differential geometry, but not in biology. I think you meant to use the word ‘evolve’. Also, you have a deep misconception about the evolutionary relationship between humans and chimpanzees.
Reply | Report Abuse | Link to thisYour misunderstanding has led you to first ask why all apes haven’t evolved into humans and second why we assume that the evolutionary direction is chimp-to-human rather than the other way around. To clarify, humans and chimps evolved from a common ancestor; neither species evolved (or evoluted) into the other. This basic concept, which all attentive, non-home-schooled, high school students should understand, reveals that your questions have emerged from your misconception.
Also, not only is the evidence from chromosomal comparison not problematic for evolution, but it actually supports common descent. If it were really true that you have read a lot about evolution, you would know that human chromosome 2 matches up with chimp chromosomes 2 and 3, supporting the principle that our ancestors at some point had 24 pairs, and then chromosomes 2 and 3 became fused at the telomeres, yielding 23 pairs. Understand this: ALL of the human and chimp chromosomes match up.
Also, if it were really true that you have read a lot about evolution, you would know to use the term ‘transitional fossils’ rather than ‘missing link’. The latter implies that there is only one and that it is missing, while the former exist to a sufficient degree to inform us that anyone who asks for examples has not been paying attention.
I suggest that you do what you claimed (falsely) to have already done: actually read about evolution. That doesn’t mean going onto creationist websites and then regurgitating the nonsense that you find there. It means going to an actual bookstore and actually purchasing an actual book on the topic, and then actually reading it. For starters, I recommend Jerry Coyne’s book entitled, “Why Evolution Is True”. When you have done this, you will no longer have to ask your initial question, which was, “What is the basis of evolution?”
I totally disagree with this view. I used agree with this view but it's illogical. This view denies our impact upon the environment. Each and every species leaves a mark upon the environment. With presence of our species, it will be a carbon dioxide footprint. This footprint will change the biology of the plant permanently. Like the microorganisms that created the oxygen we breath, the planet was changed forever. It didn't return to what is before because most chemical reactions are not reversible.
Reply | Report Abuse | Link to thiswayel ... The fusing of chromosomes 2 and 3 are well documented including the vestigial centromere. There are also a hand full of reversals. Think of DNA as file folders while the genes are the files. The chromosomes can get mixed while still functioning correctly as carriers of genes. The fusing took place some 3 million years ago and well after the separation of chimps and humans.
Reply | Report Abuse | Link to thisYes, it is possible for a being with 23 chromosomes to breed with a being with 24 chromosomes. The fusing propagates if the organisms are successful.
Evolution is driven by the environment. Organisms are selected by the environment and successful selections are amplified by reproduction. It's that simple.
There is no 'direction' for evolution where an 'end point' is a goal. All organisms are not 'trying' to evolve to become humans. Organisms that fit their environment stay the same generation after generation. Differences that are not successful are culled and do not reproduce. Variations are introduced by chromosome crossing in sexual reproduction, viral and bacteria infections, cosmic rays, and a small but measurable error rate in chromosome replication.
Evolution is an incredibly simple concept and easily proven to be true. Simple programs can illustrate how evolution works: the environment selects and reproduction amplifies. It's actually easier to evolve than to stay the same.