Distinguish whether a pair of animals are related by descent or share a common ancestor

[askingask]

So I've been reading a bit about Human ancestry and Human-Chimpanzee common ancestry, and one thing that you tend to hear a lot is the notion that humans didn't evolve from chimpanzees but rather they share a common ancestor. I've read about how this common ancestor population could have been more similar to humans/chimps or in-between.

But I don't understand how one could distinguish wether they are descendants of a shared ancestors or descendants of each other. Now I know the idea of the chimp population staying the same species for 7 million years is a bit far fetched. But then again I don't know if that is possible. Although this might be something for another question.

But to get back. What about a parent and a sibling. Shouldn't both share 50% of their DNA with me. The only way to know who's an ancestor and who shares and ancestors with you, would be to maybe look at the age right? But in the context of millions of years and gene flow between not just individuals but population, how does one distinguish?

 

[JT Smith]

What about a parent and a sibling. Shouldn't both share 50% of their DNA with me.

You're forgetting about mutation. I think that there are a significant number of single point variations that distinguish an offspring from the combined DNA of its parents. To complicate matters one's genome isn't consistent across all the cells in the body since mutations occur during development. In addition there are changes that happen over one's lifetime. So I don't think it's true that you are the simple sum of your parent's DNA.

 

[Drakkith]

But to get back. What about a parent and a sibling. Shouldn't both share 50% of their DNA with me.

You share something like 99+% of your DNA with both parents and with me and all other humans. The overwhelming majority of our DNA is identical with everyone of our species, with that small remainder determining all the differences we see in people. The 50% claim is more related to which genes you inherit, which are themselves 99+% identical to the genes from the other parent.

The only way to know who's an ancestor and who shares and ancestors with you, would be to maybe look at the age right? But in the context of millions of years and gene flow between not just individuals but population, how does one distinguish?

That would have something to do with genetic distance and how it is determined. Alas, I am well outside my knowledge base here, so I cannot contribute much more than a Wikipedia link. All I know is that the subject is very complicated overall.

 

[askingask]

So I don't think it's true that you are the simple sum of your parent's DNA.

The 50% claim is more related to which genes you inherit, which are themselves 99+% identical to the genes from the other parent.

The way I framed this was a bit shallow sorry. What I mean is that there are a few parts of the Genom that are unique to individuals for example, or concerning phylogenetics unique to a species. Usually these are non functional parts of the Genom. From these indicators scientists can derive descent of an individual or species or in other cases common descent. Really the question focuses on how that distinction is made wether two individuals/species are related by common descent or descendants of each other.

 

[pinball1970]

The way I framed this was a bit shallow sorry. What I mean is that there are a few parts of the Genom that are unique to individuals for example, or concerning phylogenetics unique to a species. Usually these are non functional parts of the Genom. From these indicators scientists can derive descent of an individual or species or in other cases common descent. Really the question focuses on how that distinction is made wether two individuals/species are related by common descent or descendants of each other.

Chimps have their own evolutionary history is the short answer.

Our Genome is about 2% coding and that 2% is about 99% similar with the Chimp, Pan troglodytes.
The entire genome is about 98% similar.

https://www.genome.gov/15515096/200...finds-chimps-humans-very-similar-at-dna-level

Other primates have had the genomes mapped also, we are closer to Chimp than the chimp is to other primates.

Do not forget we did not start classification with DNA we started with morphology so we already knew we have a close connection to other primates especially the Chimp.
That work began in 1735 with Carl Linnaeus over 200 years before Watson and Crick and then decades again before DNA technology.
Then we had what the fossil record told us, the further back in we go the more ape like and chimp like our morphology becomes.
You are aware of Lucy? Ardi? Key bone structures that indicate Ape or human traits?

Ape fossils are quite rare IRRC but worth a google.

 

[askingask]

You are aware of Lucy? Ardi? Key bone structures that indicate Ape or human traits?

Ape fossils are quite rare IRRC but worth a google.

Actually this is part of the motivation for the question. From my current understanding it is sometimes debated wether fossiles are only relatives or ancestors to modern humans.

Do not forget we did not start classification with DNA we started with morphology so we already knew we have a close connection to other primates especially the Chimp.

I know, but while morphology seems to be more open to interpretation, DNA is far more accurate.

Then we had what the fossil record told us, the further back in we go the more ape like and chimp like our morphology becomes.

Yet neanderthals are quite similar to us but not seen as our descendants, but rather they are relatives of us. Could one not say this about every fossile we dig out.

I think my question goes back to cladisitics.

 

[askingask]

Chimps have their own evolutionary history is the short answer.

Could ardi in this case not be another relative of the descendants of the chimpanzee?

 

[jim mcnamara]

Humans have > 87million SNP's which can and do affect phenotype. << short answer. Diversity: Individuals have vast numbers of unique SNPs for each gene , I've seen 57000 estimated for just a single gene. In practice, it does not matter how exact this estimate is, the result of randomly dealing out large numbers of SNPs during the formation of gametes is mind boggling.... do a google search on SNP before you get mired in "what if's"

Before you start speculating, which is a hopeless task for the human genome, learn about SNPs. Speculation not supported on the Forums

 

[BillTre]

From my current understanding it is sometimes debated wether fossiles are only relatives or ancestors to modern humans.

In reality, that which was once alive, but now is a fossil, could have been a direct ancestor to something else (like people). Or it could have died without leaving any progeny and therefore not an ancestor to anything.
This can pertain to either individuals or whole species.

Yet neanderthals are quite similar to us but not seen as our descendants, but rather they are relatives of us. Could one not say this about every fossile we dig out.

I think my question goes back to cladisitics.

The Neanderthals are an interesting example since we now know that at least some of them interbred with humans (along with the Denisovans). As a result, most humans not strictly of African ancestry carry some of their genes in their genomes. Clearly the non-human DNA came from some ancestor somewhere.

Cladistics in its basic form really likes lineages that only bifurcate. For example, branch points leading to three different species are not favored and usually "resolved" into several different possible pairs of bifurcations. This best allows cladistic methods to trace relationships.
This goes along with the common taxonomic attitude that species are always leaves (on the end of branches) on the tree of life. This view rules any species as being an ancestor to anything else. Clearly not the actual case. My view is that this is a conservative assumption made to simplify/eliminate these kinds of arguments which are not usually easy to answer.
However, cladistic approaches have been adapted to deal with lineages that anti-branch (need a better word here) back together in various ways (making reticulate or anastomosing lineages) through events like hybridization.

 

[askingask]

Humans have > 87million SNP's which can and do affect phenotype.

This view rules any species as being an ancestor to anything else. Clearly not the actual case.

Although I find the answers very informative and helpful, they don't answer the core of my question. The question was simple: how does one distinguish between common ancestry and descendants from each other. I'm not trying to speculate :(, I just want to elaborate my original question. I appreciate all of your inputs, they have greatly expanded the scope of my research.

Chimps have their own evolutionary history is the short answer.

This is the closest to an answer to my question. Could you elaborate on this?

 

[pinball1970]

Although I find the answers very informative and helpful, they don't answer the core of my question. The question was simple: how does one distinguish between common ancestry and descendants from each other. I'm not trying to speculate :(, I just want to elaborate my original question. I appreciate all of your inputs, they have greatly expanded the scope of my research.


This is the closest to an answer to my question. Could you elaborate on this?

Have a look at this article

https://www.cambridge.org/core/jour...ees-at-gombe/CBC4C65C1F6678E1C5D98422C1DF9110

 

[pinball1970]

Although I find the answers very informative and helpful, they don't answer the core of my question. The question was simple: how does one distinguish between common ancestry and descendants from each other. I'm not trying to speculate :(, I just want to elaborate my original question. I appreciate all of your inputs, they have greatly expanded the scope of my research.


This is the closest to an answer to my question. Could you elaborate on this?

The below is useful too,

https://en.wikipedia.org/wiki/Apomorphy_and_synapomorphy

 

[Drakkith]

Although I find the answers very informative and helpful, they don't answer the core of my question. The question was simple: how does one distinguish between common ancestry and descendants from each other. I'm not trying to speculate :(, I just want to elaborate my original question.

Simple question, with a complex answer I expect. One way to tell if one species is NOT directly descended from another is to look at the fossil record and see if the lineage of species A can even be traced back to species B. For example, if species B appeared 500,000 years ago, but there is evidence of a lineage from species A back to 2 million years ago where it looks like the two lineages diverged, we can be confident that A is not directly descended from B.