Tree of Life: What Creature Was at the Fork?

[Loudmouth]

I think I know what you are saying, but for clarity see if I rewrite what you said correctly:

The conclusion we are disagreeing with is that the first human compatible individual could not have come from human incompatible parents.

Whether we label them as human or not is irrelevant to the whole question.

 

[rush1169]

Whether we label them as human or not is irrelevant to the whole question.

Since the 'human' label is irrelevant, then it doesn't matter if we use the label. So, can I assume that you affirm that the two quotes are interchangeable?

The conclusion we are disagreeing with is that this first compatible individual could not have come from incompatible parents.

and

The conclusion we are disagreeing with is that the first human compatible individual could not have come from human incompatible parents.

 

[Loudmouth]

Since the 'human' label is irrelevant, then it doesn't matter if we use the label. So, can I assume that you affirm that the two quotes are interchangeable?

I affirm that it is semantics.

 

[rush1169]

If the first human gamete compatible individual born could not have been the offspring of parents that are not human gamete compatible, where did he or she come from?

 

[Loudmouth]

If the first human gamete compatible individual born could not have been the offspring of parents that are not human gamete compatible,

You have yet to show any evidence to support this "if", or why your if should even be considered.

 

[rush1169]

You have yet to show any evidence to support this "if", or why your if should even be considered.

I'm not sure I understand. You are asking for evidence that a human gamete began to exist?

 

[Loudmouth]

I'm not sure I understand. You are asking for evidence that a human gamete began to exist?

I am asking for the genetic mechanism that would prevent an indivudal who is incompatible with modern humans from giving birth to an individual that is compatible with modern humans.

 

[CabVet]

When that first individual was born that was sexually compatible with humans, given the infinite gradations between species, would it be appropriate in layman terms to call that individual the first human? Would it be scientifically appropriate?

No, it would not be scientifically accurate because this first individual would also be 100% compatible with every individual living at the same time as he was (and according to your own definition, those individuals would not be human). Scientifically, he would be an intermediate between us and what came before, and would be compatible with both.

There is no such thing as a first individual of a species. Yes, at some point a single individual became compatible with us, but morphologically he was identical in every way to our ancestors. Trying to identify the first human is equivalent to trying to find out whether the egg or chicken came first.

 

[rush1169]

I am asking for the genetic mechanism that would prevent an indivudal who is incompatible with modern humans from giving birth to an individual that is compatible with modern humans.

I don't think there is a genetic mechanism that would stop a human compatible gamete to arise from a pair that do not produce human compatible gametes. I think it's necessary for humans to exist. I guess I don't understand what evidence you're looking for.

 

[rush1169]

No, it would not be scientifically accurate because this first individual would also be 100% compatible with every individual living at the same time as he was (and according to your own definition, those individuals would not be human). Scientifically, he would be an intermediate between us and what came before, and would be compatible with both.

From a scientific standpoint a creature that can mate with two different species is what is termed the 'intermediate species'?

There is no such thing as a first individual of a species. Yes, at some point a single individual became compatible with us, but morphologically he was identical in every way to our ancestors. Trying to identify the first human is equivalent to trying to find out whether the egg or chicken came first.

So maybe the human-gamete compatibility came after the morphological transition had taken place or maybe the human-gamete compatibility came first while the individual still looked not human. But does it matter? It's my understanding that gamete compatibility is the 'gauntlet', so-to-speak, that gives substantial credence in determining what kind of animal is being discussed, regardless of physical appearance.

There must be such a thing as the first individual of a species. If there were zero humans, they are birthed one-at-a-time, and now there are many, there had to be a first. I understand it would be impossible to identify the first born of a new species, but to deny that a first had to happen is not reality.

 

[Loudmouth]

From a scientific standpoint a creature that can mate with two different species is what is termed the 'intermediate species'?

No, that's not it at all. An intermediate species is a species that has a mixture of features from two divergent taxa.

It's my understanding that gamete compatibility is the 'gauntlet', so-to-speak, that gives substantial credence in determining what kind of animal is being discussed, regardless of physical appearance.

Your understanding is wrong.

No one tries to mate modern human gametes with H. erectus fossils.

 

[rush1169]

From a scientific standpoint a creature that can mate with two different species is what is termed the 'intermediate species'?

No, that's not it at all. An intermediate species is a species that has a mixture of features from two divergent taxa.

What do scientists call a creature B that can mate with creature A and creature C such that creature A and C cannot mate?

It's my understanding that gamete compatibility is the 'gauntlet', so-to-speak, that gives substantial credence in determining what kind of animal is being discussed, regardless of physical appearance.

Your understanding is wrong.

No one tries to mate modern human gametes with H. erectus fossils.

LOL, duh!
Sexual compatibility is not used to identify animals with certainty? Just what they look like?

 

[Loudmouth]

What do scientists call a creature B that can mate with creature A and creature C such that creature A and C cannot mate?

Creature B.

Sexual compatibility is not used to identify animals with certainty? Just what they look like?

Sexual compatibility is not used. Most of the time, DNA samples are taken back to the lab and compared with DNA databases.

For fossils, what they look like is what they use since fossils do not mate, nor do they have DNA (except for very recent ones).

 

[rush1169]

Sexual compatibility is not used. Most of the time, DNA samples are taken back to the lab and compared with DNA databases.

Does DNA matching correlate to sexual compatibility?

 

[Loudmouth]

Does DNA matching correlate to sexual compatibility?

Not in any specific sense. There isn't a set percentage of homology that indicates compatibility in each and every case.

 

[rush1169]

Not in any specific sense. There isn't a set percentage of homology that indicates compatibility in each and every case.

So matching DNA does not correlate to sexual compatibility. Weird. Seems like the closer two DNA strands are to matching, the more likely to have come from sexually compatible creatures.

 

[CabVet]

So matching DNA does not correlate to sexual compatibility. Weird. Seems like the closer two DNA strands are to matching, the more likely to have come from sexually compatible creatures.

You don't know much about DNA, do you? You can have two species that have been separated for millions of years, those species could be 20% different in their DNA, yet, if brought back together they could mate and produce offspring. The opposite is also true, you can have two populations of the same species that have been separated for just a couple of generations, but if a mutation spreads in the right gene, those species can have a 99.9% identical DNA but not be able to produce viable offspring. Biology is not black and white.

I can give you examples of both if you want.

 

[rush1169]

You don't know much about DNA, do you? You can have two species that have been separated for millions of years, those species could be 20% different in their DNA, yet, if brought back together they could mate and produce offspring. The opposite is also true, you can have two populations of the same species that have been separated for just a couple of generations, but if a mutation spreads in the right gene, those species can have a 99.9% identical DNA but not be able to produce viable offspring. Biology is not black and white.

I can give you examples of both if you want.

Please and thank you!

 

[CabVet]

Please and thank you!

Take a look at this paper:

Evolutionary animation: How do molecular phylogenies compare to Mayr's reconstruction of speciation patterns in the sea?

It's a nice summary showing how the rate of gamete evolution is decoupled from the rate of genomic evolution in sea urchins. In other words, you can have species with very similar DNA that are incompatible and others with dissimilar DNA that are compatible. Let me know if you want more.

 

[Loudmouth]

Seems like the closer two DNA strands are to matching, the more likely to have come from sexually compatible creatures.

For the most part, yes. However, there is not a set amount that always correlates with compatibility. As CabVet mentions above, there is a wide range of differences that can result in incompatibility, especially when you consider chromosomal re-arrangements. For example, there aren't that many differences between horses and donkeys, but their offspring are almost always sterile. Horses and Przewalski's horses also differ in chromosomal count and have about the same amount of divergence, but their offspring are fertile. Another example is the mutation that causes dwarfism. The allele for achondroplasia is dominant, meaning that you only need one copy of the gene in order to have achondroplasia. Gametes that are both positive for the gene result in a non-viable embryo, so 25% of the time a couple with dwarfism will have a failed pregnancy, and this is just from a single mutation in a single gene.