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View Full Version : Why have there been so few Hominid species?


UnclePoo
01-10-2004, 02:40 PM
I know that evolution is the reason that Man presently walks the Earth. Through hundreds of thousands of years, we have evolved into the being that currently rules the planet. In all aspects of the word we have been an extremely successful species. With all this being so, then why have so few Hominid species walked the Earth? In all other cases of successful species the tree of evolution for these creatures branches out like a mighty Oak while our lineage more resembles the steam of a flower with only a few (unsuccessful) leaves like "Lucy" springing out from the base. There are 2500 species of mosquito currently flying around and I can only think of about 8 or so Hominids that ever walked the planet.
Has it always been in our species nature to kill off all similar competition? I know that finding proof for something like this would be impossible to prove but without some similar extremetheory, I just can't think of a reason as to why we are an exception the rules of evolution.

UnclePoo
01-10-2004, 02:53 PM
Excuse me!!
Damn Spell Check "STEM"!!

Polycarp
01-10-2004, 03:09 PM
Some orders and families speciate extensively and rapidly, in evolutionary terms (e.g, the Muriformes, the Chiroptera); some, very much less so.

There are at least five genera of extinct Hominidae (one having an extant species), and a number of species ranging from nine on up to thirty or so, depending on which authorities you subscribe to. This does not compete with rats and mice or bats, but it is certainly in a league with Canidae, Felidae, and Equidae, and not far behind Sciuridae.

Lemur866
01-10-2004, 03:19 PM
The thing is, Hominids are a pretty specific branch of the primate tree. You can't compare them to mosquitoes, or any other insects really, since insects are so species rich. The fair thing is to compare them to other large mammals, excluding rodents which are a special case among mammals..

When we look at other mammals we don't find that hominids are such a sparse branch. First of all, we have to clarify what we mean by "hominid". If we use the term the way it has been used traditionally to mean any extinct primate more closely related to Homo sapiens than to Pan troglodytes, then we have a smaller number. But really chimps, orangutans and gorillas should be included in the same family as humans if we want to have a familial ranking that compares to other mammals. So pretty much any extinct ape should be counted as a hominid, unless you want to go the other way and say that humans and human ancestors should be counted as pongids.

So given the expanded definition of Hominid, there have been dozens of hominid species. When we compare this bush to Camelids, Giraffids, Ursids, Procyonids, Suids, Equids, etc, our branch doesn't seem so sparse. There are plenty of mammals where there are only one or two extant species in their ORDER, let alone family. There is only one species of Aardvark, two species of Proboscideans, one Colugo, one Platypus, one Echidna. I would guess that if you charted a distribution of number of species per family it would resemble a poisson distribution....very many families with 0 extant members, many with 1 member, quite a few with 2 or 3, fewer with 4, only a few with 5, and less and less as you increase.

So I don't think hominids are such an outlier, expecially if you include extinct species. And I disagree that our tree is especially sparse, with only a few branch species. Look at the variety of gracile and robust australopithecines, or the various kinds of Homo that are lumped into Homo erectus or archaic Homo sapiens. If I had to bet, I would guess that very few of the hominid species that we dig up are direct human ancestors, probably MOST are on branches with no extant survivors.

So the hypothesis that hominids are uniquely intolerant of sister hominid species is not supported by the evidence.

Bosda Di'Chi of Tricor
01-10-2004, 03:19 PM
[list=1]
We have only been around for a few hundred thousand years, so there hasn't been a lot of time for us to diversify.
Species form to adapt to individual environments. But we are extremely versitile in our current form, & have little need to change to suit any given environment. After all, we're already doing quite well as it is.
We are the only species that is so very good at altering our environment to suit our needs, rather than the other way round.
[/list=1]

XT
01-10-2004, 03:40 PM
Not an expert on this subject, but isn't one of the reason for so few branches that our species, um, recombined before our subgroups speciated? We have several, um, versions, of humans (what used to be called 'races') but before they could actually speciate into separate species, humans migrated and what have you, breeding enough between distinct 'groups' that no one group became a separate species. Even, by some theories, neandarthal MIGHT have been re-absorbed into the general population through interbreeding because they were not at the point of being distinct species yet.

So, part of the answer is the nature of Humans...we are a very mobile and VERY adaptable species, that can move pretty much at will throughout the world and into varied environments. We also seem to have, as a species, a will to explore and expand.

All of the above might be completely or partially wrong...its been a long time since college anthropology classes.

-XT

'possum stalker
01-10-2004, 04:32 PM
As to why there is only one Homo species now...
In order for speciation to occur, there must be some barrier to gene flow (AKA mating). It can be physical, such as mountains, seas, etc., behavioral, like the different songs and mating 'dances' among birds, morphological (remember Cecil's joke about the "right-hand threaded" pig?), or genetic (donkey+horse= odd # of chromosomes = sterile mule. Otherwise horses and asses would collapse back together into one species.). Several hundred thousand years ago, there were multiple humanoid species, but modern humans either eliminated them (quite possible, since they were poor neighbors; a dig in Serbia found roasted modern human skulls prepared as food at a Neadertal "camp") or interbred with them (seems likely to me; some controversial "hybrids" have been found in Spain which may have both H. sapiens and Neandertal characteristics). So either we killed them, or we ARE them.

This brings up the 'ol ecological principle of competitive exclusion, which basically says that no two organisms can occupy the exact same niche indefinitely because one will ultimately outcompete the other (barring predatory, mutualistic interactions, "fugitive species," etc. hedge hedge). Heck, how many species of cunning, furless raccoons does one planet need?

Note that I'm a plant scientist, not an anthropologist. I was about to launch into some bonus speechifying about hybrid vigor, but I will spare us all the horror.

UnclePoo
01-10-2004, 06:31 PM
Originally posted by xtisme
Not an expert on this subject, but isn't one of the reason for so few branches that our species, um, recombined before our subgroups speciated? We have several, um, versions, of humans (what used to be called 'races') but before they could actually speciate into separate species, humans migrated and what have you, breeding enough between distinct 'groups' that no one group became a separate species. Even, by some theories, neandarthal MIGHT have been re-absorbed into the general population through interbreeding because they were not at the point of being distinct species yet.

So, part of the answer is the nature of Humans...we are a very mobile and VERY adaptable species, that can move pretty much at will throughout the world and into varied environments. We also seem to have, as a species, a will to explore and expand.
-XT

I am referring to how we got here and not where we are going. With the invention of the airplane (1903) replacing the ship, your correct that our species is very quickly losing the biodiversity it has acquired over the past few hundred thousands years of race isolation but, thatís another thread/story.
As a species we haven't been more mobile than any other, (excluding the past 500 years or so). Monkeys never rode horses and they are found all over the world. However when you move backward to come to the common ancestor that we share with the apes, our Hominid side then branches off with so few new species evolving that when compared with the branch that has evolved Chimp's, Gorillas, Baboons, Orangutanís etc, etc, etc and all of the other species on their side that have become extinct it is blaringly obvious that there is a major difference in the diversity of the two lines. Iím not saying that this proves or disproves anything. I am just extremely curious as to why this is the case. As far as I know, there is no other comparable species that has an ancestral line that even closely resembles the relatively straight line like that of our own. Consequently thatís why our lineage is so hard to piece together and has so many incomplete links.

'possum stalker
01-10-2004, 06:58 PM
As far as I know, there is no other comparable species that has an ancestral line that even closely resembles the relatively straight line like that of our own. Consequently thatís why our lineage is so hard to piece together and has so many incomplete links.


A cladogram for the Hominidae (http://tolweb.org/tree?group=Hominidae&contgroup=Catarrhini).

ZebraShaSha
01-10-2004, 06:59 PM
I invented time travel and went back in time and gave hominids machine guns. Then I came to the future and erased all knowledge of this. Also, I created the hula hope craze of the 50s.

HPL
01-10-2004, 07:08 PM
Originally posted by ZebraShaSha
Also, I created the hula hope craze of the 50s.

No, you didn't. Norville Barnes did.

You know, for kids.

XT
01-10-2004, 07:16 PM
From UnclePoo
I am referring to how we got here and not where we are going. With the invention of the airplane (1903) replacing the ship, your correct that our species is very quickly losing the biodiversity it has acquired over the past few hundred thousands years of race isolation but, thatís another thread/story.

Well, I don't think human type expansion is limited to the invention of the airplane. The point is, from the time Humans started spreading out they CONTINUED to spread out and migrate. There were very few barriers that humans couldn't eventually overcome...and they were able to over come them BEFORE speciation, though not before there were local adaptations obviously.

As to your monkeys statement...well, its true that they are found in many places (though not all over the world...not many monkeys in Europe or further north, ehe?). The fact that they don't ride horses or use boats is the point....THATS why there are lots of different species of monkey, while there aren't different species of humans. Monkeys spread out, became isolated populations, and eventually speciated. Humans (homo sapient) spread out, and CONTINUED to spread out, constantly moving and keeping contact with the various gene pools out there (I say 'constantly' though obviously there were times when populations DID become isolated...just not isolated long enough to speciate).

The other various species you list are the same. They spread out, became isolated, and eventually speciated, as they didn't continue to re-establish contact between the various gene pools.

Now, if you want to go back to earlier times with Homo Habilis and Australiopiticus (prolly spelled em both wrong), there WERE multiple species of hominid then...but only the Homo line persisted.

-XT

manhattan
01-10-2004, 07:24 PM
It's a bit of a fallacy that sapiens is an extremely successful species. We're one of the more successful large species. We've done OK in terms of geographic spread, and we've developed the means to defeat or limit the success of many of the other species which would pose a threat to us. But for all that, there's only 6 billion of us a full million-plus years after the first hominid walked. We're vastly outnumbered by any number of small mammals and insects, not to mention plant species and (especially) bacteria. That big huge brain we're always toting around is expensive in terms of energy requirements and dangerous or sometimes fatal to mothers. We take a very long time to mature to the point where we could live independently (if at all), we're not particularly agile or strong or hardy, and we have teensy tiny litters. Even given the advantage that brain gives us, it's kind of a wonder that there are any hominids extant.

labmonkey
01-10-2004, 08:04 PM
Originally posted by UnclePoo
However when you move backward to come to the common ancestor that we share with the apes, our Hominid side then branches off with so few new species evolving that when compared with the branch that has evolved Chimp's, Gorillas, Baboons, Orangutanís etc, etc, etc and all of the other species on their side that have become extinct it is blaringly obvious that there is a major difference in the diversity of the two lines.

Sorry for the nitpick, but one of these things is not like the others, babbons are not apes. Also chimps, gorillas, and orangs are all in the family. Humans and chimps have more in common than chimps and orangs, so as for it being a case oftheir side doing more speciating, it's kind of a sketchy assertation. Other posters have already answered the OP(lack of physical barriers, lengthy childhood and the evolution of adolesence, low number of progeny) so I won't bother. Late to the party again :smack:

Peter Doubt
01-11-2004, 02:17 AM
If the comparison is between oak trees and mosquitoes, I'd have to say the relative fecundity between those two and hominids would have to be a major factor in explaining the number of different species in their respective groups. Lemur866 and labmonkey have good points too.

Polycarp
01-11-2004, 08:49 AM
Originally posted by manhattan
It's a bit of a fallacy that sapiens is an extremely successful species. We're one of the more successful large species. We've done OK in terms of geographic spread, and we've developed the means to defeat or limit the success of many of the other species which would pose a threat to us. But for all that, there's only 6 billion of us a full million-plus years after the first hominid walked. We're vastly outnumbered by any number of small mammals and insects, not to mention plant species and (especially) bacteria. That big huge brain we're always toting around is expensive in terms of energy requirements and dangerous or sometimes fatal to mothers. We take a very long time to mature to the point where we could live independently (if at all), we're not particularly agile or strong or hardy, and we have teensy tiny litters. Even given the advantage that brain gives us, it's kind of a wonder that there are any hominids extant.

The advantage to a single altricial litter over a multiple-birth precocial litter can be explained in evolutionary terms, though I don't have the vocabulary nor the memory of how it was done. But upright posture (with resulting forelimbs capable of manipulation) combined with intelligence to reason out how to best make use of the environmentally available materials has led to human beings spreading across more of the planet, and occupying more econiches on it, than any other species save two, who co-adapted to make use of our skills and econiches to produce ones for themselves: Mus musculus and Rattus rattus.

John Mace
01-11-2004, 05:15 PM
UP wrote:
As a species we haven't been more mobile than any other, (excluding the past 500 years or so). Monkeys never rode horses and they are found all over the world. However when you move backward to come to the common ancestor that we share with the apes, our Hominid side then branches off with so few new species evolving that when compared with the branch that has evolved Chimp's, Gorillas, Baboons, Orangutanís etc, etc, etc and all of the other species on their side that have become extinct it is blaringly obvious that there is a major difference in the diversity of the two lines. Iím not saying that this proves or disproves anything. I am just extremely curious as to why this is the case. As far as I know, there is no other comparable species that has an ancestral line that even closely resembles the relatively straight line like that of our own.

Monkeys have speciated much more prolifically than apes, although the Miocene period is when the apes really had their heyday. Apes have tended to be large, rather specialized mammals, and that might be one reason.

But you are making an assumption that the other extant apes come from lines that speciated more than the hominid line. That is actually incorrect, or rather we are lacking in knowledge about the speciation of the extant-ape ancestors. The hominid line is fairly well documented, but not much is known is about the evolution of chimps or gorillas or orangs. What data do you have about the bushiness of their evolutionary trees as compared to the hominid line?

And keep in mind that the assignment of a fossil to a particular species is, to a large degree, subjective. Anthropologists like Walpoff would put all the Homo fossils in the same species. Anthropoligists like Tattersall might designate there being 7 or more Homo species: habilis, erectus, ergastor, heidelbergensis, antecessor, rudofensis, neanderthalensis. And that doesn't count all the Australopithicines.

So, how many proto-chimp species do you think there were?

manhattan
01-11-2004, 05:47 PM
Originally posted by Polycarp
The advantage to a single altricial litter over a multiple-birth precocial litter can be explained in evolutionary terms, though I don't have the vocabulary nor the memory of how it was done. IIRC, it was related exactly to the problem of our expensive brains -- too many little expensive brains crying out at once and parents couldn't take care of them all. Even if they could feed them food, they couldn't feed them the lessons and lore that make it worthwhile to have the brains in the first place. But perhaps my memory is in error, and, like you, I'd welcome a refresher in this regard and think the OP would forgive the short hijack.

But upright posture (with resulting forelimbs capable of manipulation) combined with intelligence to reason out how to best make use of the environmentally available materials has led to human beings spreading across more of the planet, and occupying more econiches on it, than any other species save two, who co-adapted to make use of our skills and econiches to produce ones for themselves: Mus musculus and Rattus rattus. You, you.. mammalist! ;)

Yeah, we've done pretty well for a mammal. And in particular, we've made short work of many other mammals which might have challenged us for geographic primacy. And by almost any fair definition we're a reasonably successful species.

But we and fish and all kinds of other guys would have to bow to the arthropods -- in particular insects. We give different taxia to beetles, much as space aliens might or might not give different taxia to us, but they're more numerous, almost as well spread, have a broader range of adaptations and a longer lineage than we do. And even then, that betrays me as an animalist -- the bacteria are kicking all our butts in the success department.

The same adaptations which have allowed us our geographic success have subsequently made us extremely interconnected -- always a risk for a species. If/when the disease comes along that those adaptations can't solve wipes us all out because we're flying to see each other, insects and bacteria will feed on our corpses for a while until they're gone and they have to eat something else. Even when the next asteriod comes along and wipes out 90% of species (probably including ours), the bacteria will still be here.

We're successful, but let's not let it go to our heads. We might be first or tied for first in the division, but it's a big league. ;)

pantom
01-11-2004, 08:07 PM
Ah yes, if it ain't the diseases, it's the asteroids. Or famine.
If we don't get it in the wash, Mother Nature'll catch us in the rinse. And then there's the spin cycle...

Blake
01-14-2004, 01:28 AM
There is only ...two species of Proboscideans
Three species, two genera. One Asian, one African savanna and one African forest.

one Echidna.
Two species, two genera. One short beaked and one long beaked