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Author Topic:   Evolution of Humans even more complex than thought
DBlevins
Member (Idle past 276 days)
Posts: 652
From: Puyallup, WA.
Joined: 02-04-2003


Message 1 of 7 (634277)
09-20-2011 12:38 PM


The evolution of human morphology had generally been thought to be one that was more straightforward, with the more gracile forms replacing the more robust forms, such as Neanderthals and the recently discovered Denisovians (with little to no hybridization). Re-analysis of the Iwo Eleru cranium along with other fossils found in the Congo, appears to indicate a more complex picture of human evolution, with archaic features retained among populations living more recently than previously believed.

Additional link to BBC News report for a quick review of the paper.


Replies to this message:
 Message 2 by Percy, posted 09-20-2011 1:20 PM DBlevins has responded
 Message 4 by RAZD, posted 09-20-2011 3:54 PM DBlevins has not yet responded

  
Percy
Member
Posts: 13362
From: New Hampshire
Joined: 12-23-2000
Member Rating: 1.9


Message 2 of 7 (634281)
09-20-2011 1:20 PM
Reply to: Message 1 by DBlevins
09-20-2011 12:38 PM


Discoverers of new hominin fossils are a notoriously self-promoting group, often claiming their find is a direct human ancestor, and the most important one, too, so it's refreshing to see a group announce a discovery confirming what has actually been long believed by less attention-seeking hominin researchers, that despite the claims of all the extravagant self-promoters in the human origins field, our ancestry is likely a very complex bush. The new discovery may indicate that human evolution is more complex than we had evidence for, but not more complex than we thought.

--Percy


This message is a reply to:
 Message 1 by DBlevins, posted 09-20-2011 12:38 PM DBlevins has responded

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 Message 3 by DBlevins, posted 09-20-2011 2:33 PM Percy has acknowledged this reply

    
DBlevins
Member (Idle past 276 days)
Posts: 652
From: Puyallup, WA.
Joined: 02-04-2003


Message 3 of 7 (634298)
09-20-2011 2:33 PM
Reply to: Message 2 by Percy
09-20-2011 1:20 PM


It seemed to me that the dominant thoughts about Human evolution were that it was much more 'linear' and it hasn't been until recently that studies have shed some more light onto the complexity of our lineage. Certainly the biggest objection I had had with the identification of some individual hominin fossils as separate species has been the geologically recent bottleneck effect on our variation, which to my mind created a more narrow interpretation of what constituted H. Sapiens. With the inclusion of these recents finds, others in West Africa and the recent findings concerning hybridization among Neanderthals and Denisovians, it appears that there is a stronger multi-regional facet to the OOA hypothesis.
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RAZD
Member
Posts: 16133
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 2.0


Message 4 of 7 (634310)
09-20-2011 3:54 PM
Reply to: Message 1 by DBlevins
09-20-2011 12:38 PM


and older Homo saps
Hi DBlevins, thanks.

The skull was compared with a modern and an 140,000 year old skull, all Homo sapiens,

The older skull is comparable to the two Homo sapiens idaltu skulls found in Ethiopia that are ~160,000 years old.

http://www.berkeley.edu/.../releases/2003/06/11_idaltu.shtml

Interestingly, there is a link at the bottom of your page to discusses another pair of skull from Ethiopia that are now considered older than idaltu -- 196,000 years:

http://news.bbc.co.uk/2/hi/science/nature/4269299.stm

quote:
They suggested the specimens must be between 104,000 and 196,000 years old - but with some additional climate evidence on ancient flooding in the region, the team was able to show the Omo finds were actually very close to the 196,000-year mark.

Dr Chris Stringer, from London's Natural History Museum, worked on the skulls more than 20 years ago. He told BBC News: "I was of the opinion that Omo I was a modern human - Omo II seemed much more primitive. So, from my point of view I thought Omo II might be older than Omo I.

"But it seems that they are about the same age and that shows that the populations in Africa at that time were very variable. They show different mixtures of primitive and modern characteristics."


Again showing variation in the population.

Enjoy.


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
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RAZD
Member
Posts: 16133
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 2.0


Message 5 of 7 (634315)
09-20-2011 4:48 PM
Reply to: Message 3 by DBlevins
09-20-2011 2:33 PM


Hi again DBlevins,

It seemed to me that the dominant thoughts about Human evolution were that it was much more 'linear' and it hasn't been until recently that studies have shed some more light onto the complexity of our lineage.

People like to think of species as linear, however when you look at subspecies and variations between populations, it is self evident (imho) that a bushier arrangement is appropriate, maybe even more of a weave when subspecies interbreed and produce hybrids.

Personally I think there will always be an impetus to branch away from the parent lineage, particularly when new eco-systems are involved that cause variation on the selection process. Until such branching reaches speciation, the picture will be shrubby rather than linear.

Certainly the biggest objection I had had with the identification of some individual hominin fossils as separate species has been the geologically recent bottleneck effect on our variation, which to my mind created a more narrow interpretation of what constituted H. Sapiens.

Because it happened during the time H.sapiens was already established as a species. Don't know how it affected the H.neanders (anyone know?) ...

With the inclusion of these recents finds, others in West Africa and the recent findings concerning hybridization among Neanderthals and Denisovians, it appears that there is a stronger multi-regional facet to the OOA hypothesis.

Be careful here. Multi-regional, IIRC, was originally about H.sapiens arising independently in several locations (an aspect I have always had trouble with -- such a scenario should result in different species or subspecies not the same one), whereas what we have here is some rather limited hybridization between hominid populations. I would expect that many hybrid offspring may have been infertile, thus reducing the impact of such individuals on the overall populations. After all some mules are fertile and can produce offspring with horses and donkeys.

http://en.wikipedia.org/wiki/Mule

quote:
here are no recorded cases of fertile mule stallions.[citation needed] A few female mules have produced offspring when mated with a purebred horse or donkey.[9][10] Since 1527 there have been more than 60 documented cases of foals born to female mules around the world.[9] There are reports that a mule in China produced a foal in 1984.[11][12] In Morocco, in early 2002, a mare mule produced a rare foal.[9] In 2007 a mule named Kate gave birth to a mule son in Colorado.[13][14] Blood and hair samples were tested verifying that the mother was a mule and the colt was indeed her offspring.

So I would not be surprised to see similar small numbers of offspring from hybrids when the genetic divergence had almost reached isolation levels.

Enjoy


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
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This message is a reply to:
 Message 3 by DBlevins, posted 09-20-2011 2:33 PM DBlevins has responded

Replies to this message:
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DBlevins
Member (Idle past 276 days)
Posts: 652
From: Puyallup, WA.
Joined: 02-04-2003


Message 6 of 7 (634361)
09-21-2011 12:33 AM
Reply to: Message 5 by RAZD
09-20-2011 4:48 PM


People like to think of species as linear, however when you look at subspecies and variations between populations, it is self evident (imho) that a bushier arrangement is appropriate, maybe even more of a weave when subspecies interbreed and produce hybrids.

I should have been more clear, but I was speaking of the literature and instruction. H. sapiens evolution was considered to be more 'linear' in the sense that it was thought that we evolved from one species, which evolved from a previous one, and so on. Few were advocating either hybridization or a multi-regional type gene flow. It has been a fairly recent development that hybridization and gene flow has been found to have occured during our evolution, and this recent paper helps 'bush' out our evolution from the stricter linear one.

Because it happened during the time H.sapiens was already established as a species. Don't know how it affected the H.neanders (anyone know?) ...

Again, I should have been clearer. I used to be of the impression that the relatively small amount of morphological variation in humans today, was due to the recent development bottleneck. The greater variation in the DNA of previous populations (before the bottleneck) would have been reflected in their greater morphological variation as compared to us today, and these populations should have been thought of as belonging in our species.

Be careful here. Multi-regional, IIRC, was originally about H.sapiens arising independently in several locations (an aspect I have always had trouble with -- such a scenario should result in different species or subspecies not the same one)

Which is NOT what I was taught in College, even in 1988. My understanding, and the textbook from that time that I still have, states that the Multiregional hypothesis posits that enough gene flow occured to allow the populations to evolve together, and NOT that they evolved independently.

I would expect that many hybrid offspring may have been infertile, thus reducing the impact of such individuals on the overall populations. After all some mules are fertile and can produce offspring with horses and donkeys.

As far as H. sapiens is concerned, certainly that is a possibility, but I should point out an example that fits a bit closer to home, so to speak. From a 2001 article by Clifford Jolly:

Another source of phylogenetic uncertainty is the possibility of gene-flow by occasional hybridization between hominins belonging to ecologically and adaptively distinct species or even genera. Although the evidence is unsatisfactorily sparse, it suggests that among catarrhines generally, regardless of major chromosomal rearrangements, intersterility is roughly proportional to time since cladogenetic separation. On a papionin analogy, especially the crossability of Papio hamadryas with Macaca mulatta and Theropithecus gelada, crossing between extant hominine genera is unlikely to produce viable and fertile offspring, but any hominine species whose ancestries diverged less than 4 ma previously may well have been able to produce hybrid offspring that could, by backcrossing, introduce alien genes with the potential of spreading if advantageous. Selection against maladaptive traits would maintain adaptive complexes against occasional genetic infiltration, and the latter does not justify reducing the hybridizing forms to a conspecific or congeneric rank. Whether reticulation could explain apparent parallels in hominin dentition and brain size is uncertain, pending genetic investigation of these apparently complex traits.
(my bold)
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RAZD
Member
Posts: 16133
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 2.0


Message 7 of 7 (634428)
09-21-2011 12:59 PM
Reply to: Message 6 by DBlevins
09-21-2011 12:33 AM


Hi again DBlevins,

Which is NOT what I was taught in College, even in 1988. My understanding, and the textbook from that time that I still have, states that the Multiregional hypothesis posits that enough gene flow occured to allow the populations to evolve together, and NOT that they evolved independently.

But that would still have problems generating a homogeneous species, rather than isolated subspecies or varieties that have a low or little interest in cross-mating. Personally, I just don't see it as one big hybrid zone. Now I could be wrong, and this may be the best explanation for races (which are essentially varieties) that I've seen. I look at the Greenish Warbler for an example of how I see it operating:

http://www.zoology.ubc.ca/~irwin/GreenishWarblers.html

quote:
Greenish warblers (Phylloscopus trochiloides) inhabit forests across much of northern and central Asia. In central Siberia, two distinct forms of greenish warbler coexist without interbreeding, and therefore these forms can be considered distinct species. The two forms are connected by a long chain of populations encircling the Tibetan Plateau to the south, and traits change gradually through this ring of populations. There is no place where there is an obvious species boundary along the southern side of the ring. Hence the two distinct 'species' in Siberia are apparently connected by gene flow. By studying geographic variation in the ring of populations, we can study how speciation has occurred. This unusual situation has been termed a 'circular overlap' or 'ring species'. There are very few known examples of ring species.

  • The geographical isolation of P. t. viridanus from P. t. lulowi is not complete and
  • there is a zone of hybridization between them, with some limited gene flow.
  • The gene flow is limited because we do not see the two varieties becoming homogenous.
  • This holds for the other hybrid zones as well.
  • Assuming the best scenario for gene flow, we can assume that there is negligible selection in the hybrid zones, because they are continually being supplied from both sides.
  • Selection does occur within each variety population and this selection will tend to homogenize each variety population to fit their ecology, so
  • this will tend to filter out some gene flow from other varieties.
  • The geographical isolation of P. t. viridanus from the other varieties is complete, so
  • the only way that gene flow can occur between P. t. viridanus and P. t. trochiloides is thru the filter of P. t. lulowi and selection in that population.
  • From P. t. viridanus to P. t. obscuratus any gene flow is thru the additional filter of P. t. trochiloides and selection in that population.
  • Finally, from P. t. viridanus to P. t. plumbeitarsis any gene flow is thru the additional filter of P. t. obscuratus and selection in that population.
  • P. t. viridanus to P. t. plumbeitarsis behave as distinct species and do not interbreed: the songs and coloration are too different to be recognized as potential mates.

There is a map of gene flow in human populations that looks similar to the Greenish Warbler in spreading out into different ecologies.

http://partners.nytimes.com/...genetics-evolution.1.GIF.html

quote:
The most detailed human family tree so far available is one constructed over many years by Dr. Douglas C. Wallace and his colleagues at the Emory University School of Medicine in Atlanta. Dr. Wallace's tree is based on mitochondrial DNA, tiny rings of genetic material that are bequeathed only by the egg cell and thus through the maternal line.

A counterpart tree for men, based on analysis of the Y chromosome, has been prepared by Dr. Peter A. Underhill and Dr. Peter J. Oefner of Stanford Universit

Population geneticists believe that the ancestral human population was very small -- a mere 2,000 breeding individuals, according to a calculation published last December. But the family tree based on human mitochondrial DNA does not trace back to the thousand women in this ancestral population. The tree is rooted in a single individual, the mitochondrial Eve, because all the other lineages fell extinct.

The same is true of the Y chromosome tree, a consequence of the fact that in each generation some men will have no children, or only daughters, so the number of different Y chromosomes may steadily diminish, even if the population stays the same size.


This image shows an OOA pattern for the main gene flow and diversification. It doesn't address the possibility that some of those genetic variations come from hybridization with other hominids.

Again, I should have been clearer. I used to be of the impression that the relatively small amount of morphological variation in humans today, was due to the recent development bottleneck.

There is some evidence of this:

http://www.bbc.co.uk/nature/life/Human

Go to the link near the bottom for

quote:
The new batch

Human beings are phenomenally successful animals. But our species, Homo sapiens, once came close to outright extinction.


And then page down to

quote:
Humans: from near extinction to phenomenal success

With over 6 billion people living in the world today, human beings are a phenomenally successful animal. But our species, Homo sapiens, once came close to outright extinction.

Clues from genetics, archaeology and geology suggest our ancestors were nearly wiped out by one or more environmental catastrophes in the Late Pleistocene period. At one point, the numbers of modern humans living in the world may have dwindled to as few as 10,000 people.


There is more information there about the possible causes.

Enjoy.


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.


Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)

This message is a reply to:
 Message 6 by DBlevins, posted 09-21-2011 12:33 AM DBlevins has not yet responded

  
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