Tree of Life: What Creature Was at the Fork?

[rush1169]

I see many "Tree of Life" depictions. Some look like trees, others are circles, and some look like my March Madness bracket. What I can't seem to find is an illustration that shows/labels what the creature was before the split.

For example, you may find a tree that shows a chimp on the tip of one branch and a human on the tip of another. If you follow both branches to their intersection, there is nothing there. The illustrations imply that at some point before humans and chimps split, they shared a common ancestor. What was that ancestor? Does it have a name? Is there a fossil? Artists rendering?

There must be millions of intersections where two creatures evolved from a common ancestor, but I've never seen intersections labeled with what it was before the split into two creatures. Why is that?

 

[Papias]

Because even if we have a likely fossil, etc, we can't know for sure if that creature is exactly at the split, or if a cousin of that creature is (because fossils don't have birth certificates). That, coupled with the very conservative nature of science to avoid saying something that isn't well established, keeps those branch points bare.


However, I think that this has been overly conservative. After all, we know a lot about the ancestors at many of those branc points. To look at one example you gave, we have a pretty good idea of what the last common ancestor between chimps and humans looked like, at about 6 million years ago, based on the fossils just before and just after that point. It looked a lot like a chimp, much more so than a human. Similarly, the last common ancestor of all birds looked a lot like archeopteryx, etc.

With that in mind, I think it would be very helpful for an informed biologist to make estimated ancestor pictures for many of the main trunk splits. Doing so, with the added caveat that these are estimates, I think could help all of us understand our family tree much better.

Thanks for pointing this out.

Papias

 

[Kylie]

I see many "Tree of Life" depictions. Some look like trees, others are circles, and some look like my March Madness bracket. What I can't seem to find is an illustration that shows/labels what the creature was before the split.

For example, you may find a tree that shows a chimp on the tip of one branch and a human on the tip of another. If you follow both branches to their intersection, there is nothing there. The illustrations imply that at some point before humans and chimps split, they shared a common ancestor. What was that ancestor? Does it have a name? Is there a fossil? Artists rendering?

There must be millions of intersections where two creatures evolved from a common ancestor, but I've never seen intersections labeled with what it was before the split into two creatures. Why is that?

There's no way we can ever know if a particular animal was the actual species that was split into two. Also, given that the split takes place over a period of time, it gets even harder. After all, it's not like there's one parent creature who gives birth to two offspring and one offspring is Species A and the other is Species B. it's more like there are two populations of Species A that are divided somehow. It could be a river that is redirected after a flood, or a small group that is isolated (a number of insects washed to an island on a tree, for example). Now we have two populations of the same species living in two different environments. With different environmental pressures acting on them, the two populations will diverge. So you can see, the split itself is one that takes place over many generations.

 

[rush1169]

There's no way we can ever know if a particular animal was the actual species that was split into two.

we have a pretty good idea of what the last common ancestor between chimps and humans looked like, at about 6 million years ago, based on the fossils just before and just after that point. It looked a lot like a chimp, much more so than a human.

We have no 'hard' intersection data, and never will. It seems like there should be hard data somewhere for some creature at some point in time through whatever scientific studies that could conclusively demonstrate a creature that resides at the intersection, but maybe not.

We know that before there were chimps and humans there were creatures that looked more like a chimp than a human (ie before the split). We know they look more like chimps because there are no human-like creatures in that era. Time passes and now we find human remains (ie after the split). It follows that since chimp and human DNA and morphology are relatively similar and there was a time there were chimps (or chimp-like) and no humans and later humans appeared, it must be that humans are a result of an evolved chimp-like creature. Is that generally how the inclusion of a branch is determined?

 

[Papias]

rush wrote:

We know that before there were chimps and humans there were creatures that looked more like a chimp than a human (ie before the split). We know they look more like chimps because there are no human-like creatures in that era. Time passes and now we find human remains (ie after the split). It follows that since chimp and human DNA and morphology are relatively similar and there was a time there were chimps (or chimp-like) and no humans and later humans appeared, it must be that humans are a result of an evolved chimp-like creature. Is that generally how the inclusion of a branch is determined?

I think that's pretty close for many splits. It will depend on which split you are talking about as to whether or not there are a lot of fossils on either side of the split or not. Other data (DNA, comparative anatomy, biochemistry, etc.), confirm what the splits are (what the family tree is - as in who evolved from who), but much of that doesn't show the form of the last common ancestor (LCA).

For instance, all those and more lines of evidence show that humans and, say, penguins shared a common ancestor. The fossil record shows what this ancestor was like. It turns out that the LCA between penguins and us was something like hylonomus (and lived around that time or a little earlier). Hylonomus looked a lot like a modern lizard, and not like modern penguins nor humans. This is an example of where a good estimate can be made of what the ancestor at the branch point would have looked like, and I agree that a family tree with creatures like that at the branch points would make our family history easier to understand.

KTS wrote:

After all, it's not like there's one parent creature who gives birth to two offspring and one offspring is Species A and the other is Species B. it's more like there are two populations of Species A that are divided somehow. It could be a river that is redirected after a flood, or a small group that is isolated (a number of insects washed to an island on a tree, for example). Now we have two populations of the same species living in two different environments.

Right. Though at the same time, recognize that even in your scenario above, there was at least one mother A, who had kids, some of which were on one side (and some on the other) after the river split the population. Thus, that one mother did have one (or more) child who was ancestral to all of A, and one child (or more) that was ancestral to all B. This is simply a logical neccessity.

Papias

 

[Loudmouth]

I see many "Tree of Life" depictions. Some look like trees, others are circles, and some look like my March Madness bracket. What I can't seem to find is an illustration that shows/labels what the creature was before the split.

The nodes of a cladogram are not species. Instead, they are synapomorphies which are the shared features of the species on that branch. The "tree of life" represents relationships, not direct lines of ancestry.

For example, you may find a tree that shows a chimp on the tip of one branch and a human on the tip of another. If you follow both branches to their intersection, there is nothing there.

The node is there, and it is represented by the features and DNA that humans and chimps share.

The illustrations imply that at some point before humans and chimps split, they shared a common ancestor. What was that ancestor? Does it have a name? Is there a fossil? Artists rendering?

We would need DNA from all of the ape populations at that point, and DNA samples from subsequent generations in order to determine which of those populations is an ancestor to living populations. We don't have that DNA, so there is no way to determine if those fossils have living descendants or not.

What we can do is try to determine how closely related living species are, and try to determine how closely fossil species are by comparing their morphology to other fossil species and to living species.

There must be millions of intersections where two creatures evolved from a common ancestor, but I've never seen intersections labeled with what it was before the split into two creatures. Why is that?

Because fossils do not come with birth certificates, to put it more simply.

 

[Kylie]

KTS wrote:

Call me Kylie. The only reason I am KTS is because Kylie was already taken.. So I used my initials.

Right. Though at the same time, recognize that even in your scenario above, there was at least one mother A, who had kids, some of which were on one side (and some on the other) after the river split the population. Thus, that one mother did have one (or more) child who was ancestral to all of A, and one child (or more) that was ancestral to all B. This is simply a logical neccessity.

Of course, this should not be taken to mean that the offspring that was ancestral to Species A was a different species to Species B.

 

[Papias]

Loudmouth wrote:

The nodes of a cladogram are not species. Instead, they are synapomorphies which are the shared features of the species on that branch.

yes, but.....

The "tree of life" represents relationships, not direct lines of ancestry.

That depends on the tree diagram being looked at.

Yes, some are cladograms, as you pointed out. But others truly are family trees, with the nodes showing common ancestors. This type is easier to conceptualize - it is what we think of easily. That's why it's the type I prefer. After all, there really was a history like that, with ancestors making a huge family tree. Since it's real, and since it's what many of us will understand, why not try to show it?

That's the type I think rush is referring to. Here is an example.

rush, this one shows some ancestors at the branch points, too:

Tree of Life: What is Phylogeny

Kylie wrote:

Of course, this should not be taken to mean that the offspring that was ancestral to Species A was a different species to Species B.

Right.


In fact, had the same sisters/brothers been on the opposite sides of the river, they may well have led to species B and species A, instead of species A and species B.

Hmmm. Maybe that ended up just being confusing. Anyway, I agree.

Papias

 

[rush1169]

The reason we can't define the creature at the chimp/human divergence is because the change happened so gradually? To expand, let's say subset chimp group was relocated and isolated. This generation would ultimately become human, but the change from chimp to human took hundreds of thousands (?) of generations to complete. Each change was probably nearly imperceptible, but if we were to find 100K generations buried sequentially and intact, we would be able to see the transition (ie make a 'flipbook' of the fossils, so to speak). Is that about right?

To compound the creature-identification-at-the-node problem, at what point would/could anyone say that the chimp is no longer a chimp, but now a human? Surely not at the first generation post-separation. Nor the 10th. If it took 100K generations to go from chimp to human, then maybe around the 75th thousandth generation one would be seeing more human than chimp. Maybe generation 74,999 would be the creature to place at the node. Does that sound about right?

 

[Kylie]

The reason we can't define the creature at the chimp/human divergence is because the change happened so gradually? To expand, let's say subset chimp group was relocated and isolated. This generation would ultimately become human, but the change from chimp to human took hundreds of thousands (?) of generations to complete. Each change was probably nearly imperceptible, but if we were to find 100K generations buried sequentially and intact, we would be able to see the transition (ie make a 'flipbook' of the fossils, so to speak). Is that about right?

To compound the creature-identification-at-the-node problem, at what point would/could anyone say that the chimp is no longer a chimp, but now a human? Surely not at the first generation post-separation. Nor the 10th. If it took 100K generations to go from chimp to human, then maybe around the 75th thousandth generation one would be seeing more human than chimp. Maybe generation 74,999 would be the creature to place at the node. Does that sound about right?

You ever hear about the paradox of the pile?

Imagine a pile of sand. Countless grains, all piled together. Anyone you ask would agree that it is a pile. Now take away a single grain of sand. Everyone will still agree that it is a pile. Continue doing this. Take away a single grain, and ask if it is still a pile.

Now, eventually you are going to be left with only one grain of sand. If you ask the people, they will say, no, that isn't a pile of sand. But at what point do you find that it is a pile of sand before you take the grain of sand away, and not a pile once the single grain is removed?

It's the same here. The tiny changes each generation are like the removal of a single grain. Each one by itself is practically unnoticeable, but eventually all those little changes add up. There was never a point where the animal population stopped being a human ancestor and became a human. It simply became less and less ancestor-like and more and more human-like until it became what we are today.

 

[rush1169]

When Animal A evolves into Animal B, the process is very slow and over hundreds of thousands of generations. For example, Chimplike (A) into Human (B). Would it be likely that Animal C would have existed as a intermediate creature that evolved from A but ultimately became B at some point during the process of A to B?

Expanding the pile of sand analogy: Let's say a pile of sand (A) will evolve into a pile of grass (B). About 1/2 way into the evolution, the sand grains have green tails (C). Now you have a pile of Sass. It's clearly not a pile of sand nor a pile of grass.

The paradox of the pile did contribute clarity to the slow transformation of creature A to creature B, but because it was only a reduction in populate exercise, it didn't help with the issue that when you finally reach the final grain of sand, it's no longer sand.

In the chimp-like to human evolution, we should expect to find a creature (C) that exhibits characteristics of both creatures (A&B). Unless all the morphological differences change at the same rate, it seems we should find a creature that features a subset of fully human characteristics and a subset of fully chimp-like characteristics. In other words, unless the brow, ears, mouth, jaw, hands, hips, hair, eyes, feet, skull, brain, diet, etc all evolved simultaneously at the same rate, we should find a creature (C) that is fully human in at least one aspect, fully chimplike in another, and remaining differences would exhibit something between the start point and end point.

Maybe all those characteristics did evolve simultaneously and at the same rate? If so, we'd never see a creature with at least one fully human feature and at least one fully chimp feature, but should still see one that is more-or-less exactly between the two. As I partake in this brain exercise, it seems that if all the changes were taking place simultaneously, it is as if the creature knew what it was to become when it started and that can't be correct.

Continuing my though process: There was a time when the chimp foot changed into a human foot. Did the slight stepwise progression through thousands of generations 'trigger' the change in leg, knee, hip, muscular, and vascular structures? Not to say that a foot mutation started the process, but some beneficial mutation occurred that must have caused the change in supporting components. Those changes in supporting components could have only been considered a beneficial mutation only after the first beneficial mutation. In other words, only after a beneficial mutation occurred in one facet could the change of supporting facets be considered beneficial rather than destructive. What good would an upright posture be if the foot wasn't able to accommodate?

ETA: I hope this post is not misinterpreted as a Crockoduck declaration. I'm interested in exploring the reason why there are no definitively labeled nodes on a tree of life depiction (my original question). The reason given is because "there is no way to ever know." That answer goes against my current logical rendition of "how it all went down". I'm trying to gain clarity by fleshing out what seemingly had to have happened if a creature A is to become creature B over thousands or millions of generations.

 

[Kylie]

When Animal A evolves into Animal B, the process is very slow and over hundreds of thousands of generations. For example, Chimplike (A) into Human (B). Would it be likely that Animal C would have existed as a intermediate creature that evolved from A but ultimately became B at some point during the process of A to B?

Yes. To put it in a slightly different way, if Species A evolved into Species Z, it will evolve though Species B, C, D...W, X, and Y before getting to Species Z. It must do this - it is impossible for an individual of Species A to give birth to an individual of Species Z. Species B through to Species Y must be there.

Expanding the pile of sand analogy: Let's say a pile of sand (A) will evolve into a pile of grass (B). About 1/2 way into the evolution, the sand grains have green tails (C). Now you have a pile of Sass. It's clearly not a pile of sand nor a pile of grass.

Yep. When something evolves from one thing to another, it will pass through an intermediary stage. Of course, if you saw that intermediary stage, it wouldn't be obviously a transitional. it would be just as valid a species as anything else. Evolution isn't aiming towards some specific goal. If you found Species A through to Species M (with Species M being the present day species), you couldn't predict the qualities of Species Z (which will not be around for a long time). All you can do is say that the lineage that has lead from Species A to Species M will continue, resulting in new species in the future. You might be able to make some educated guesses about the near future, but the further into the future, the less accurate you will be. For example, I can confidently predict that there will still be birds flying in 1 million years. But I can't predict what they will be like. But I sure can't say if there will be birds in 500 million years. And even if they are, I couldn't tell you what they'll be like.

The paradox of the pile did contribute clarity to the slow transformation of creature A to creature B, but because it was only a reduction in populate exercise, it didn't help with the issue that when you finally reach the final grain of sand, it's no longer sand.

Ah, but the analogy wasn't about changing from sand into a non-sand thing. It was about changing from a pile into a non-pile.

In the chimp-like to human evolution...

I'm going to stop you there. Humans didn't evolve from chimps. Both Humans and chimps evolved from a common ancestor. There was a species, say Species 1A, and it was split into two populations. One group became Species 2 and the other group became Species B. Species 2 could be Humans and Species B would be chimps. Species 1A was the common ancestor of chimps AND humans.

we should expect to find a creature (C) that exhibits characteristics of both creatures (A&B). Unless all the morphological differences change at the same rate, it seems we should find a creature that features a subset of fully human characteristics and a subset of fully chimp-like characteristics. In other words, unless the brow, ears, mouth, jaw, hands, hips, hair, eyes, feet, skull, brain, diet, etc all evolved simultaneously at the same rate, we should find a creature (C) that is fully human in at least one aspect, fully chimplike in another, and remaining differences would exhibit something between the start point and end point.

Not quite. To a degree, yes, but it's a mistake to think that fully human features just popped up where they were never there before. Let's say the common ancestor had Trait A, and Humans have Trait B. We wouldn't find that Trait B began popping up. We'd find that Trait A gradually, over many generations, became more and more like trait B.

Maybe all those characteristics did evolve simultaneously and at the same rate? If so, we'd never see a creature with at least one fully human feature and at least one fully chimp feature, but should still see one that is more-or-less exactly between the two. As I partake in this brain exercise, it seems that if all the changes were taking place simultaneously, it is as if the creature knew what it was to become when it started and that can't be correct.

No. Evolution doesn't aim towards a set goal. If you could travel through time and watch the change from chimp/human ancestor to Human, you'll see that the population had traits that were quite ancestor like but these traits slowly became more and more human like.

it's like when they morph people's faces from one to the other. Let';s say you morph from Tom Hanks' face into Anne Hathaway's face. You could look at a particular feature, let's say the eyes. They would start out as Tom's eyes, but as the face slowly morphed, the eyes would become less and less Tom-like and more and more Anne-like.

Continuing my though process: There was a time when the chimp foot changed into a human foot. Did the slight stepwise progression through thousands of generations 'trigger' the change in leg, knee, hip, muscular, and vascular structures? Not to say that a foot mutation started the process, but some beneficial mutation occurred that must have caused the change in supporting components. Those changes in supporting components could have only been considered a beneficial mutation only after the first beneficial mutation. In other words, only after a beneficial mutation occurred in one facet could the change of supporting facets be considered beneficial rather than destructive. What good would an upright posture be if the foot wasn't able to accommodate?

The posture became slowly more and more upright as the foot became more and more human like.

if we take it one step at a time, it could be something like...

The foot becomes a tiny bit more human like, on average, throughout the population. This slightly more human foot might give little benefit by itself, but any individual which could stand slightly more upright would see a slightly larger benefit. The benifit might be tiny, but as long as it was there, it would be enough to give any individual that had it an evolutionary advantage.

ETA: I hope this post is not misinterpreted as a Crockoduck declaration. I'm interested in exploring the reason why there are no definitively labeled nodes on a tree of life depiction (my original question). The reason given is because "there is no way to ever know." That answer goes against my current logical rendition of "how it all went down". I'm trying to gain clarity by fleshing out what seemingly had to have happened if a creature A is to become creature B over thousands or millions of generations.

You'd be very interested in ring species. Have a look at "The Salamander's Tale" in "The Ancestor's Tale" by Richard Dawkins. It explains how what we call "species" are really little more than arbitrary points along a continuum of constantly changing individuals.

 

[rush1169]

Hi Kylie - thank you for your dialog with me

I'd like to dig a little deeper if I may.

I think we agree when looking at the chimphuman ancestor to chimps and humans that there were a large succession of creatures between chimphuman and human. Probably hundreds of thousands. So talking about the chimphuman as being the common ancestor to chimps and humans is valid, but there were many creatures in between the chimphuman and human. It follows that chimphuman may be the LCA of chimps and humans, but it's not the immediately prior creature to humans. In other words, I'd like to examine more closely the creature that was just before human and forget about the chimp side of the branch.

In a tree-of-life, there is a node marking the LCA between chimps and humans. That node is a creature, but then we see a line extending from the node that ends with human. That line must represent hundreds of thousands of different creatures as we move from the chimphuman node to the end-point of human?

Whatever creature-A population was immediately before human-B, wouldn't one of them be required to birth a human? And that first human be required to have the ability to fertilize either A or Bs? Certainly that creature was very, very close to actually being human in all facets, but wasn't quite there for some reason. But, at some point a non-human had to birth a human. There had to be a beginning human. Those infinitesimally small steps towards human at some point had to stop being Creature-A and become human.

 

[Kylie]

Hi Kylie - thank you for your dialog with me

I'd like to dig a little deeper if I may.

Sure thing.

I think we agree when looking at the chimphuman ancestor to chimps and humans that there were a large succession of creatures between chimphuman and human. Probably hundreds of thousands. So talking about the chimphuman as being the common ancestor to chimps and humans is valid, but there were many creatures in between the chimphuman and human. It follows that chimphuman may be the LCA of chimps and humans, but it's not the immediately prior creature to humans. In other words, I'd like to examine more closely the creature that was just before human and forget about the chimp side of the branch.

No worries.

In a tree-of-life, there is a node marking the LCA between chimps and humans. That node is a creature, but then we see a line extending from the node that ends with human. That line must represent hundreds of thousands of different creatures as we move from the chimphuman node to the end-point of human?

That's right.

Whatever creature-A population was immediately before human-B, wouldn't one of them be required to birth a human? And that first human be required to have the ability to fertilize either A or Bs? Certainly that creature was very, very close to actually being human in all facets, but wasn't quite there for some reason. But, at some point a non-human had to birth a human. There had to be a beginning human. Those infinitesimally small steps towards human at some point had to stop being Creature-A and become human.

Not quite.

The definition of "species" is that if two individuals (of opposite genders, generally) can interbreed and produce a fertile offspring, then the two parents are the same species. Therefore, my mother and my father were the same species, because they were able to have me, and I have proved that I am fertile (by producing an offspring of my own). In one of Richard Dawkins' books, he says that you can take any Individual A who is alive today and carry them back in time 1000 years. They would be able to interbreed with any of our ancestors from that time. We then pick up an individual B, take B back in time a further 1000 years (2000 years before today) and they would be able to interbreed with the individuals of that time. We can take an individual C and do the same thing, and then with D and E and so on. Every single time, the individuals we take back will be able to interbreed with individuals from 1000 years previously. This is because the changes during that 1000 years are so small that any two individuals 1000 years apart will be called the same species. But if we make enough of these 1000 year jumps to go back a million years (it would take a thousand jumps), we'd find that even though each individual could breed with the others only 1000 year away from it, the 1 million years ago individual would be so different from our first Individual A that the two wouldn't be able to interbreed.

Another way of thinking of it is to imagine you have a row of squares, all lined up. At one end, they are red, but as you go along, they change gradually to blue. If the row is 1000 squares long, you can look at any two squares next to each other and they will seem to be the same colour. They would be like our different individuals separated by 1000 years. The tiles are the same colour, and the individuals are the same species. But as you go along the row, the colour gradually changes until the first and last square are different colours, just as the first and last individuals are different species, even though there is no obvious difference between the individuals we met at consecutive stops during our trip back in time.

Hope that's clear. it's a bit of a confusing topic.

 

[rush1169]

If we take a current-day-human (CDH) and go back 1,000 years we find it can breed with 100% of the population. We should continue back in time until we find that our CDH can breed with 99% of the population. Go back further and our CDH can breed with 98%. Further back and 97%. Continue until only 1% is compatible with CDH. Maybe 1% represents 10,000 individuals. Keep going until there was born that first individual that could successfully reproduce within the definition of species with CDH.

If we go back 1,000 years and find 100% compatibility and at 1M years find 0% compatibility, there must be a time where we would find either a gradual compatibility transition (as illustrated above) or a toggle between 0% and 100%. In the former case, a single individual. In the latter, a single generation (composed of individuals). It seems the former fits best. In either case, there was a first-born 100% compatible creature that could reproduce either human or non-human when mated with the non-human, immediate ancestor to human. Right or wrong?

 

[Kylie]

If we take a current-day-human (CDH) and go back 1,000 years we find it can breed with 100% of the population. We should continue back in time until we find that our CDH can breed with 99% of the population. Go back further and our CDH can breed with 98%. Further back and 97%. Continue until only 1% is compatible with CDH. Maybe 1% represents 10,000 individuals. Keep going until there was born that first individual that could successfully reproduce within the definition of species with CDH.

If we go back 1,000 years and find 100% compatibility and at 1M years find 0% compatibility, there must be a time where we would find either a gradual compatibility transition (as illustrated above) or a toggle between 0% and 100%. In the former case, a single individual. In the latter, a single generation (composed of individuals). It seems the former fits best. In either case, there was a first-born 100% compatible creature that could reproduce either human or non-human when mated with the non-human, immediate ancestor to human. Right or wrong?

Not quite. More like it would be able to mate with any member and have a 99% chance of producing a fertile offspring. The way you phrased it made it sound like there were some individuals it would have a 0% chance with and others it would have a 100% chance with. If there was a creature that could reproduce 100% with both humans and non-humans, then they would all be humans. A species, after all, is generally defined as two individuals who are able to produce fertile offspring. If that 100% compatible creature is Creature B, and it could reproduce properly with Creature A as well as Creature C, then all three creatures must be the same species.

It's probably best to stick with the coloured square analogy.

 

[Sectio Aurea]

The thing to keep in mind here is that one species will NEVER give birth to another species. A new species is formed when a group of common species seperate into two or more groups. Then over a very long period of time one (or more) of these groups is no longer able to reproduce with the other group(s). There is no definitive point in time when this occurs.

 

[Kylie]

The thing to keep in mind here is that one species will NEVER give birth to another species. A new species is formed when a group of common species seperate into two or more groups. Then over a very long period of time one (or more) of these groups is no longer able to reproduce with the other group(s). There is no definitive point in time when this occurs.

Exactly. If you remember my coloured square analogy, with the squares going gradually from red to blue, there's no point where there's a blue square next to a red square.

 

[mzungu]

The thing to keep in mind here is that one species will NEVER give birth to another species. A new species is formed when a group of common species seperate into two or more groups. Then over a very long period of time one (or more) of these groups is no longer able to reproduce with the other group(s). There is no definitive point in time when this occurs.

Indeed and that is why all life forms are transitional.

A good example of evolution and transitional would be:

Imagine if a human baby represented species A and at old age represented species Z. At what point can we place B,C,D,......etc. In fact if we wanted to be precise we would have to place representations like A1aaaa then A1aaab, etc.
Yes some lifeforms evolve(d) faster while others slower; this depends on the pressures the life form experiences in its environment. Cockroaches have evolved very little simply because they adapted so well that there was very little pressure for them to evolve. Today Cockroaches are faced with a new pressure (insecticides) and this will invariably lead them to evolve into "super roaches" capable of being immune to such toxins. Yet one may see no visible difference between a normal roach and a super roach.

 

[rush1169]

For the following text, consider 'human' to be a creature than can procreate with a current-day-human (CDH) and non-human to be a creature that cannot procreate with a CDH.

There was a time when the population of humans was zero. There were creatures that were very similar in many ways, but were not human. For there ever to be one human, a non-human is required to birth a human. There were once zero humans, so to get to a billion humans, we can't jump from zero humans to 1,000 humans without first having 100 humans. Likewise, we can't jump from zero humans to 100 humans without first having 10 humans. It follows, there had to be a first human. That first human is required to have been birthed by a non-human. Now that one human exists, there cannot be a second human unless that first human can birth a human offspring when fertilized by a non-human.

I realize I just said the same thing as my immediate prior post just in different words. Three replies tell me that what I've written is wrong. So, I'd like to talk more about the scenario.

For clarity, I'd like to summarize each response, so please let me know if I didn't compose my summary accurately.

KTS says that if CDH can mate with the first human and the first human can mate with the last non-human, then all three are human. It seems you started with 3 humans instead of 1.

Sectio says that there was a group of non-humans. That group separated from each other and after a long time one group could not mate with the other group. It seems you started with non-humans and ended with two groups of non-humans that couldn't breed (divergent evolution).

mzunga says assume human baby is Creature A and when that baby is old we call it Creature Z. For every microsecond that elapses, that human is slightly different so we could call it Creature A1, A2, A3. You could further timeslice to approach infinity and label each of those as a different creature. It seems you illustrated generic evolution using the imagery of a baby growing old.

If there were once zero humans and now humans are abundant, there had to be a first. It's not clear to me why that statement is false. Thank you for any additional details you can provide so I can understand why there was never a first human.