Homo floresiensis evolutionary path

Factual Questions
1

From this NOVA / sc1ence NOW interview with Jared Diamond, noted evolutionary biologist:

And:

This seems to imply that the very next generation needs to be smaller which of course won’t happen. Clearly the population was viable since they survived (and flourished).
Elephants and other animals aside, it seems humans who were bigger would demand and get their share of resources.

What is the biological advantage of being just a tad bit smaller that would ensure your genes -to the exclusion of biggies- would prevail and eventually lead to a smaller race? I understand how a 30-pound population would fare better but how would they beat out the bigger humans to get there?

Steven

2

Diamond was giving a generalized argument from a population perspective, rather than the actual likely course of the evolutionary history of H. floresensis.

Assuming the original colonists were normal-sized, food would have been limited on an island that small. Smaller individuals, with lower caloric requirements, would be more likely to survive periodic famines. The reduction in the average body size of the population, and the increase in number of individuals the island could support, would have been gradual, probably over a period of thousands of years.

3

[QUOTE=Don’t fight the hypothetical]
This seems to imply that the very next generation needs to be smaller which of course won’t happen. Clearly the population was viable since they survived (and flourished).
[/quote]

An important point to recognise here is that the original colonists were being introduced to a totally unexploited ecosystem. When they arrived the elephants, birds and other animals would have been so quite they could have literally walked up to them and hit them on the head. We see this historcially on every island that people land on that has never known humans. In addition to that there would have been large amounts of plant food, coastal foods and so forth that were totally unexploited. So, far from the very first generation needing to be smaller the original population wou’d have had a massive glut of food that would have easily supported massive individuals,.

However this sort of situation can only ever last for a short period of time. Inevitably resources are overexploited and any surviving animal species wise up. It is then that the environment starts favouring smaller individuals. However the process of conversion from resoucres rich to resource poor environment doesn’t happen overnight, or even in a single generation. The entire process was slow enough to enable natuarl selection to start working on the human population.

One important point that you are overlooking here is that humans are both extremely territorial and extremely social.

Within a clan no individual can “demand” anything because hunter-gatherer clans are egalitarian. An individual could try stealing food from his own clan members but will be banished or killed as soon as he tries reagrdless of his size.

A clan of large individuals could try using their size advantage to approriate resources from neigbouring clans, but with stone age technology a clan of 50 small people will always repel any attempted inbvasion from a clan of 5 large people. A pygmy may not be able to throw a spear quite as hard as someone 7 feet tall, but still more than hard enought o kill anyone it hits.

So as you can see large individuals have no inherent advatage at all if that comes at the cost of numbers.

Quite simply because smaller people require fewer resources. All things being equal small person starves to death more slowly. It really is that simple. By extension a clan consisting of smaller indivdiauls can support more individuals in exactly the same territory.

4

[QUOTE=Colibri]
Diamond was giving a generalized argument from a population perspective, rather than the actual likely course of the evolutionary history of H. floresensis.
[/QUOTE]
He also ignores the incidence of island gigantism in which herbivore or omnivorous scavenger species, lacking a predator, will sometimes grow to an abnormally large size. Island gigantism (like other many other evolutionary oddities that occur on allopatrically diverged species) seems to be an evolutionary dead end–when competing or predatory animals are intruduced, gigantic species, like flightless birds, are at an extreme disadvantage–but as long as isolation remains in effect large size is viable.

As indicated by previous arguments, smaller size requires less resources, and at times when resources are scarce, the effect on survivability can be exaggerated, quickly eliminating larger members of the species. However, it’s inadvisiable to attempt to reduce evolutionary viability to one single factor; selection favors the organisms that are in general more adapatable and capable of reproduction. Smaller size might permit faster reproduction (less gestation, faster maturation, et cetera) offering a statistical advantage, or other factors unforseen factors could be at work. Never underestimate the complexity of natural selection; in comparison, particle physics is a pamplet of possibilities.

Stranger

5

[QUOTE=Blake]
All things being equal small person starves to death more slowly. It really is that simple. By extension a clan consisting of smaller indivdiauls can support more individuals in exactly the same territory.
[/QUOTE]

So are you saying that given two humans of of relatively equal proportions and equally fed a 150 pound person will starve faster than a 30 pound person?

6

[QUOTE=Blake]
We see this historcially on every island that people land on that has never known humans.
[/QUOTE]

Aren’t you assuming that Flores was an island at the time? Sea levels have been very much lower, and the region is geologically active. Homo Floresiensis doesn’t seem to have had - or maybe it lost - boatbuilding skills.

It is truly sad that there are none alive today. We can only hope that one day, we’ll discover some hidden valley and find them.

7

[QUOTE=Don’t fight the hypothetical]
So are you saying that given two humans of of relatively equal proportions and equally fed a 150 pound person will starve faster than a 30 pound person?
[/QUOTE]

Of course. A 30 pound dog eats less food than a 150 pound dog. At 200 pounds, I eat much more food than my 40 pound daughter. An elephant eats more than a goat. A lion eats more than a housecat. A large animal needs more food than a small animal.

But a curious pattern of island species is that large animals tend to get small, while small animals tend to get big. So you get dwarf Maltese elephants, dwarf Wrangell Island mammoths, small Sumatran tigers compared to mainland tigers, gigantic Mauritius flightless pigeons (dodos), giant Komodo monitor lizards, and so on.

8

[QUOTE=Stranger On A Train]
Island gigantism (like other many other evolutionary oddities that occur on allopatrically diverged species) seems to be an evolutionary dead end–when competing or predatory animals are intruduced, gigantic species, like flightless birds, are at an extreme disadvantage–but as long as isolation remains in effect large size is viable.
[/quote]

Hmm, I’m not sure how you came to this conclusion but would be interested in hearing about it.

Island species generally fare poorly when pitted against the biota of the entire rest of the planet, but is independent of gigantism and is entirely attributable to probability. The rest of the world got more throws of the dice and has a correspondingly higher chance of producing the superior design.

However as far as island giants being especially at risk I’m not seeing it. Large species are often at risk simply because they have smaller populations, but certainly when I look at islands like New Zealand or Aldabra the giant species don’t seem to have suffered disproportionately to the large. Giant crickets, giant tortoises and so forth have all survived while numerous average species have vanished.

I’d really like to know how anyone managed to conclude that island giants are more at risk than any other island species. Intuitively I would say that is not the case.

[QUOTE=Don’t fight the hypothetical]
So are you saying that given two humans of of relatively equal proportions and equally fed a 150 pound person will starve faster than a 30 pound person?
[/QUOTE]

Yes, as others have pointed out you can readily ascertain this yourself by looking at the food intake of a child and an adult, or of a petite woman and a large man. The larger the animal the more mass it needs to maintain and that maintenance requires food.

There are of course some exception to that rule, such as under conditions of extreme cold or conditions of extremely seasonal food abundance. But in general a large animal will starve faster than a small one, hence the reason so many island species are dwarves.

[QUOTE=Quartz]
Aren’t you assuming that Flores was an island at the time? Sea levels have been very much lower, and the region is geologically active. Homo Floresiensis doesn’t seem to have had - or maybe it lost - boatbuilding skills.
[/quote]

As far as boatbuilding skills we really can’t say. We know from the situations in Southern Australia and especially Tasmania that people isolated on small islands very rapidly lose most of their technology, including boat building. Even if the ancestors of H. floresiensis were master boatbuilders the technology would have been completely lost within a few thousand years.

As for whether Flores was an island, the fact that so many species were and remain to a large extent genetically distinct from neighbouring islands suggest very strongly that it has always been isolated. IIRC the channel separating it form the nearest island is several km deep which would mean it was always separated..

9

[QUOTE=Blake]
Hmm, I’m not sure how you came to this conclusion but would be interested in hearing about it.

Island species generally fare poorly when pitted against the biota of the entire rest of the planet, but is independent of gigantism and is entirely attributable to probability. The rest of the world got more throws of the dice and has a correspondingly higher chance of producing the superior design.

However as far as island giants being especially at risk I’m not seeing it. Large species are often at risk simply because they have smaller populations, but certainly when I look at islands like New Zealand or Aldabra the giant species don’t seem to have suffered disproportionately to the large. Giant crickets, giant tortoises and so forth have all survived while numerous average species have vanished.

I’d really like to know how anyone managed to conclude that island giants are more at risk than any other island species. Intuitively I would say that is not the case.

[/QUOTE]

They are large, which is a disadvantage for a herbovore, since it is more difficult to escape and hide. And they have evolved to the larger size, without evolving any means of defence against predators, since they don’t need them on the island. So this would cause them to be disproportionally disadvantaged, isn’t it?

10

[QUOTE=mr. jp]
They are large, which is a disadvantage for a herbovore, since it is more difficult to escape and hide. And they have evolved to the larger size, without evolving any means of defence against predators, since they don’t need them on the island. So this would cause them to be disproportionally disadvantaged, isn’t it?
[/QUOTE]

No.

If an island has no predators then no species will have any mechanism of defense against predators. While in theory smaller species might be able to hide they won’t actually do so if they have never needed to avoid predators. As under conditions where animals have no experience of predators large size is a distinct advantage. Kiwis for example have managed to survive where numerous other NZ birds have been exterminated because their eggs are simply too large for a rat or possum to eat. While the birds themselves are equally defenseless against nest predators simply being large provides an inherent advantage.

And of course if an island does have predators to propel animals to develop defences such as running, hiding and fighting then large animals aren’t at any particular disadvantage. They may not be able to hide as well, but they can both run and fight more effectively than a smaller individual.

I’m still not seeing any reason why island giants should be any more at risk, and I still cna’t think of any evidence to support the position. Certainly a great many giant island species continue to survive and thrive where many thousands of ‘normal’ sized island species have become extinct.

11

[QUOTE=Blake]
No.

If an island has no predators then no species will have any mechanism of defense against predators. While in theory smaller species might be able to hide they won’t actually do so if they have never needed to avoid predators.
[/QUOTE]

Well, they can learn to try and hide before they are extinct, can’t they? And then the large size disadvantage will kick in.

My line of thinking is, in order for a herbivore to escape, it needs to outrun the predator, fight it off, hide or climb a tree. Species evolving in a predator-less environment have no incentive to evolve good fighting or running skills. And the larger species can’t hide, but the smaller can.

12

[QUOTE=mr. jp]
Well, they can learn to try and hide before they are extinct, can’t they? And then the large size disadvantage will kick in.
[/quote]

If we assume they can learn to hide before they are extinct why can’t they also learn to fight or run away before they are extinct? And the large size advantage will then kick in since larger animals can run faster and fight harder. IOW if animals can learn defensive behaviours then all animals can learn defensive behaviours, not just small aninmals. As a result large animals should be no more prone to extinction than small animals.

But this leads us back the original problem. If the large species of an island have no incentive to evolve good fighting or running skills then the small species will likewise have had no incentive to evolve good climbing or hiding skills. And the smaller species can’t run or fight, but the larger can.

Let me put it this way. If I introduced foxes to an island populated by naive capybaras and naive house mice I would expect the mice to fare far worse than there giant cousins, not better.

13

[QUOTE=Blake]
If we assume they can learn to hide before they are extinct why can’t they also learn to fight or run away before they are extinct? And the large size advantage will then kick in since larger animals can run faster and fight harder. IOW if animals can learn defensive behaviours then all animals can learn defensive behaviours, not just small aninmals. As a result large animals should be no more prone to extinction than small animals.
[/QUOTE]

They can learn to try and defend themselves, or to try and run away. But whereas for hiding they only need to be small, the two other methods require appropriate biology. They need fast legs, or they need some sort of defence tools. The will in itself is not enough. For example, the gazelle has a body structure that is able to attain high speeds, the rhino has a horn and the porcupine has sharp spines . The dodo didn’t have any of these things.

That’s not entirely fair, is it? The fox is a predator used to mice-sized prey. If we introduced pumas to the environment, then the mice would do better.

14

[QUOTE=mr. jp]
… for hiding they only need to be small, the two other methods require appropriate biology.
[/quote]

That isn’t even remotely true. To hide an animal needs to know how to sit motionless when a predator approaches, it needs camouflage, it needs to know how to spot a predator. It needs those and a whole slew of other biological modifications. I’m not sure where you got the idea that mice or moths hide by simply sitting still but it isn’t even remotely true.

Yes, and mice need to build dens, and chameleons can change colour, and moths are active only at night and monkeys hide in trees. And the dodo didn’t have any of these things either. So what exactly is your point? Hiding requires just as many adaptations as fighting or evasion.

But gazelle survive because the young are camouflaged and have evolved to lie completely still in long grass. Rhinos survive because the young are extremely precocial and are able to follow their mothers from the moment they are born. Porcupines survive because they build subterranean dens in which to hide their altricial young. All the species you list have a whole slew of adaptations dedicated to predator evasion (and in the casse of porcupines and gazelle entirely contingent on hiding). How long do you think these things would survive if the young simply wandered around alone out in the open savanna?

I think perhaps you don’t appreciate just how many adaptations most species have for avoiding predators. Let us just say that your idea that hiding consists of nothing more than sitting still has no basis in reality.

I specifically chose foxes because they are broad spectrum predators and happily hunt gazelle fawns, mice, fish, seals and wallabies. They will eat almost any size food source you care to name if they can get ahold of it, including prey much larger than themselves.

The problem is that your entire argument seems to boild down to “In conditions in which large creatures are disadvantaged by exposure to large predators that can’t be fought off and that can for some reason only see large animals, large animals will be disadvantaged”. All you’ve done is re-state the original assertion and added a couple of unsupportable assumptions. You’ve provided no coherent argument or examples of why large species would be more at risk.

Rather than continuing to point out the obvious flaws in statements such as “hiding consists of nothing but sitting still” I’m going to simply ask for some evidence. Can you provide any evidence t support your claim that an animals can hide by simply sitting still without starving to death because it never moves? Can you provide any evidence that giant island species are more likely to be exterminated by novel predators than small?

15

[QUOTE=Blake]
That isn’t even remotely true. To hide an animal needs to know how to sit motionless when a predator approaches, it needs camouflage, it needs to know how to spot a predator. It needs those and a whole slew of other biological modifications. I’m not sure where you got the idea that mice or moths hide by simply sitting still but it isn’t even remotely true.

[/QUOTE]

Yeah, ok, you are right. Thanks for explaining things.

16

[QUOTE=Blake]
Hmm, I’m not sure how you came to this conclusion but would be interested in hearing about it.

Island species generally fare poorly when pitted against the biota of the entire rest of the planet, but is independent of gigantism and is entirely attributable to probability. The rest of the world got more throws of the dice and has a correspondingly higher chance of producing the superior design.

However as far as island giants being especially at risk I’m not seeing it. Large species are often at risk simply because they have smaller populations, but certainly when I look at islands like New Zealand or Aldabra the giant species don’t seem to have suffered disproportionately to the large. Giant crickets, giant tortoises and so forth have all survived while numerous average species have vanished.

I’d really like to know how anyone managed to conclude that island giants are more at risk than any other island species. Intuitively I would say that is not the case.
[/QUOTE]
Gigantized species, like other highly specialized species produced by extreme allopatric speciation, are certainly prone to failure in competition with other species in general; however, giants represent a particular extreme that makes it difficult for them to adapt. A typical island gigantized species occurs due to a lack of predation (virtually all gigantic species are either herbivores or insectivores, verging to scavengers, but not primary predators) and minimal interspecies competition. Hence, it makes sense, assuming an adequate supply of food and no other counterindicating factors, to be as large as possible; anyone who’s worked with cattle knows that the largest bull gets the best thistles and the most lovin’, and will run off smaller or weaker bulls.

However, gigantism is often a balance between selective pressure for large size and biomechanical limitation for skeletal support and locomotion. This places the species at an extrema from which it is difficult to “evolve back” to a more moderate size, particularly if it has adopted physiological mechanisms to support said gigantism. Even in isolation, a gigantic species might eat itself out of house and home; in competition with a smaller species, and especially in an environment in which the conditions which initially favored gigantism have changed (i.e. less food, greater predation, et cetera) a species of more moderate or even diminished size is generally more adaptable and will flourish at the expense of the giant species.

Despite the examples you list, the vast bulk of gigantic species fare very poorly even with faced with only a moderate adaptive challenge or competition, and it’s rare (I think) that a species classified as gigantic–that is, directly derived from a previous species with essesntially all phenotypical features intact and enlarged–continues onto a new, distinct lineage of species. Hence, the claim that gigantism is an evolutionary dead end. Derivative species of larger size certainly does happen, but this is generally accompanied by other significant modifications in phenotypes which preclude the classification of “gigantic”.

Stranger

17

Everyone so far has assumed the the colonization was a discrete event. Even in the OP it was stressed that without a small enough size to avoid inbreeding and a stable population size the population could be wiped out.

Humans have been on this earth for a staggering amount of time compared to the historical record. What’s to say that Flores was not colonized multiple times, and when the hobbits got there, the remaining population was either wiped out from famines or inbreeding, or wiped out to a sufficient extent that they couldn’t resist the masses of smaller folks?

So, it could have been evolution of successful colonizing events, selecting for the most successful one, rather than a gradual evolution on Flores proper.

18

[QUOTE=Blake]
Yes, as others have pointed out you can readily ascertain this yourself by looking at the food intake of a child and an adult, or of a petite woman and a large man. The larger the animal the more mass it needs to maintain and that maintenance requires food.
[/QUOTE]

-Yes, smaller mass requires less fuel but I thought the problem was one of making it through lean times. Surely if food was always abundant, there would be no need for miniaturization.

I guess I’m wrong but it seems that if you and your daughter were to become stranded on an island, it would be her that would starve first.
Steven

19

Because my daughter would be less adept at finding food? Maybe, but I’d give my daughter food even if it meant I would starve to death.

But suppose we were each on an island with one mango tree, and that mango tree provided 1000 calories worth of mangoes every day. Clearly, I’d starve to death on 1000 calories a day, but my daughter could survive. Repeat the experiment with lots of people of various sizes, and only those who were small enough to survive on 1000 calories a day would be represented in the next generation.

Of course it doesn’t work like that, but it certainly isn’t unknown for species to face famines, and those members of the species that require less food preferentially survive those famines. And a species confined to a small territory (such as an island) is more likely to have larger and more frequent swings between feast and famine. Thus, dwarfism of large species.

20

[QUOTE=Ludovic]
Humans have been on this earth for a staggering amount of time compared to the historical record. What’s to say that Flores was not colonized multiple times, and when the hobbits got there, the remaining population was either wiped out from famines or inbreeding, or wiped out to a sufficient extent that they couldn’t resist the masses of smaller folks?
[/QUOTE]

A total lack of evidence, Ockham’s Razor and no plausible explanation of how that could occur say that it didn’t happen.

H. floresiensis shows all the signs of being derived from H. erectus. Yet we’ve never found any dwarfed H. erecrtus populations anywhere else in SE Asia depsite SE Asia having one of the better hominid fiossil records. And we’ve no evidence of normal sized erectus on Flores. So positing that florisiensis migrated to Flores from the larger Indonesian islands has no evidence to support it, and positing that it displaced an extant erectus community is also totaly without support.

The SE Asian fosil record for that period is suficiently complete that we can say that if floresiensis did exist on the larger Indonesian islands it must have done so alongside normal sized erectus populations. So now we have lost all our explanatory power because we’ve got mixed populations and need some sort of mechanism for sympatric speciation. We’re now multiplying entities and still haven’t explained why floresiensis became dwarfed in the first place. We need to accept that floresienis and erectus could exist side by side on the larger Indonesian islands for eons in order for there to be a sizable poopulation of floresiensis for these postulated repeated colonisations. We therefore need to also accept that Floresienis wasn’t swamped by erectus (or vice versa) despite prolonged contact, which means we have to accept that the species simply were not competing to any significant degree. Each must have occupied a unique ecological niche with minimal overlap. But that in turn then forces us to further multiply entities and posulate some mechanism by which the species started competing as soon as they interacted on Flores.

If there was a shred of evidence for any of this, or a plausible mechanism by which the species could remain distinct without competing on the mainland it would be worth considering. But as it is Ockham’s Razor demands we reject it.