Skip to 0 minutes and 15 seconds We have already met our relatives. We know that we have to establish the evolutionary similarities. We have to trace the tree of evolution that includes our relatives, bonobo, chimp, gorilla, orang and humans; and to be able to define the similarities that have a sense in evolution. We can use morphological, we can use molecular evolution. But the main point is that if we use morphological evolution, we do not have a clock of morphological change. Something that we do have for molecular evolution. So is there a model of evolution? Is there a clock of change? And the answer is yes, if we look at the molecular evolution.
Skip to 1 minute and 15 seconds Here we can begin seeing some of the classical studies that have been done, using chromosomes, blood groups, immunological studies, sequence of proteins, and DNA. We are going to see very quickly, and we are going to focus in the last one, which is the one that is going to give us the final response to our questions. Many years ago, Yunis published a paper in which he made the comparative analysis of the chromosomes of humans and apes. In this case, it was very nice to see that the similarity in the structure of chromosomes was very high, extremely high. Even though we knew before that the number of chromosomes were different, but why were different the number of chromosomes? Very easy.
Skip to 2 minutes and 17 seconds Because chromosome 2 in humans in reality is part of two different chromosomes in the Apes. So in our ancestors, in the ancestors just of humans, what we had is a fusion of two chromosomes that gave rise to our chromosome number two. But the structure is very, very, very similar. And seen at this level of definition, there are just a few differences that can be seen between the structure of the chromosomes among the species. This was the first view, saying “wow, we are very, very similar; much more than we thought we were morphologically” The analysis of blood groups for a given moment, historically, also gave some information saying the other species, also half the ABO blood groups;
Skip to 3 minutes and 24 seconds but the question is: okay the other species may have these antigens in the red cells, but the answer is that doesn’t tell us a lot. So they have also ABO blood groups, but nothing else. So this is not very relevant. And then came several immunological studies.
Skip to 3 minutes and 46 seconds Here the point was very, very interesting in the following sense: They took -the researchers-, they took antigens from the human blood, and they injected that to a rabbit; the rabbit produced antibodies and then it was possible to measure the reaction of these antibodies to the block of different species. To be able to measure the amount of cross-reactions of the rabbit antibodies, and the blood of comparable species. The more reaction, the closer you are. Humans with humans is the maximum, and then with the other species the amount of cross reaction is lower and lower, and it’s possible to stablish an amount of reaction between pairs of species.
Skip to 4 minutes and 49 seconds And here came one of the most interesting results that was in the 80s of the last century, in which it was possible to see, first, that humans are very, very, very close to both chimps and gorillas; and also it was proposed at the time that the divergence was very small in the human branch. The idea then, here, is that the amount of differences between chimp and gorillas were similar to any of them with humans. And this was something that was shocking at the time.
Skip to 5 minutes and 31 seconds When looking at protein sequences -in this case we have the hemoglobin- what we can see is that there are lots of similarity, but when looking at different proteins, what was seen is that the amount of change depends on the specific protein. In some proteins there is lots of change, in others there is no change; depending on the strength of selection. So it doesn’t tell us a lot on the evolutionary process of the species. And then it came the DNA studies. With the DNA studies we had initially very, very interesting results with the experiments of DNA hybridization. Sibley and Ahlquist began these experiments, and King and Wilson published a very interesting paper in 1975.
Skip to 6 minutes and 24 seconds Mary-Claire King always has been saying that this has been a main paper in the history of our own understanding. The idea is very simple, the DNA is double-stranded and we know very well that when we hit the DNA, the two strands separate. And they separate depending on the amount of cross links that exist between the two.
Skip to 6 minutes and 54 seconds Then, the experiment is the following: let’s separate the two strands, and then let’s make hybrid DNAs of one strand, human and the strand with another; and the amount of links will depend on the similarity between the two DNA sequences. It’s an indirect way of ascertaining the amount of difference. So, the idea then is possible to measure the amount of similarity calculating at which temperature you need to hit the DNA to get the separation between the two strands. In this case, is possible to make the analysis of all pairs of species. Humans-chimps, chimps-gorilla, gorilla-human and so on. The results here came out as something really, very, very, very interesting.
Skip to 8 minutes and 2 seconds The main points: humans are very close to other species, and very close to chimps within the Apes. So this idea that humans is not a separate group from all the Apes came with this kind of results. And again, the amount of separation was small; meaning that it postulated that there is a very young age of separation of humans. And then the last kind of analysis, the last kind of data, is looking at DNA sequence, comparing DNA sequences. Here we have a huge amount of possibilities, the idea is very simple. To infer the amount of change, since the last common ancestor of any two species.
Skip to 9 minutes and 9 seconds So you have the DNA sequence of one species, the DNA sequence of another species, and you count the number of differences for these regions. In general, we go into regions that do not code for protein, to have a large number of changes, because selection has not been cleaning up the amount of variation, and in this case what we have to do is to have lots of information, and we can look at any part of the genome. Remember that we have 3 billion base pairs of the genome to look at, of which most of it is non-protein coding, meaning that they can be used for that.
Skip to 10 minutes and 0 seconds If we look at this, at the differences between the pairs of species, we are going to see that the more distant two species are, the more number of mutations occur. There are theoretical models in which we can infer not only what we see, but infer what has been happening in the evolutionary history of the species, and then we can recognize not only the number of difference but the number of mutations that has happened in the history of the species. If we have a look at the mitochondrial genome in a classical study by Pascal Reynier, we can see, in this case, something that was astonishing at the time.
Skip to 10 minutes and 55 seconds We see here again that humans are close to chimp, and then gorillas are more different and orangutans much more;
Skip to 11 minutes and 8 seconds and here in this work for the first time we saw something very, very interesting: that the amount of diversity within humans is extremely small. These are sequences of mitochondrial DNA, from many individuals from different species, and look at the human cluster; in which we can see that the amount of differences among that many humans is a tiny fraction of the diversity we find in chimpanzees, for example.
Skip to 11 minutes and 43 seconds This was mitochondrial DNA, but the point has been to go into the DNA of the whole genome, and in this case what we are going to see are the results of a paper that was published in 2013 by Tomàs Marquès-Bonet, in which we have 81 whole genome sequences from this species, in which you can you can see the amount of data, the amount of variation found. And from that it was possible to trace the evolutionary tree of the species. This is what today stands as the best view.
Skip to 12 minutes and 32 seconds In this case, what we have is not only the species, but also for chimps –well, for chimps we have the subspecies-, for gorillas we have-the mountain gorillas is not considered here it was considered later- and for orangs we have the two populations that most people consider two species. We can recognize, you can trace back the evolutionary history and the time of divergence of subspecies. The case of chimp is very nice, we can see the separation of chimps from bonobos some around 1 million years ago; and then clearly, our closest relative, and this is already final, is the chimpanzee. Or better said, the cluster of chimpanzee and bonobo.
Skip to 13 minutes and 30 seconds Then, we have the merging of gorillas, a small amount of diversity between the two populations of gorilla, and later we have the orangutans, also with the small amount of differentiation between the two species of orang. So we could say that nowadays, thanks to this view we have been able to trace back, to reconstruct the temple and mode of evolution of humans. So the position of humans in nature at the light of evolution, only with the molecular methods we have been able to obtain a clear image of the length of the branches and the structure of the tree. We humans are a recent species, no more than six million years old.
Skip to 14 minutes and 25 seconds Our closest relatives are chimpanzees and bonobos, with the general similarity very high; in the order of 99% if we consider the point variations. So we have this very strong relationship to the other species and again, interesting, very strong genetic similarity within humans. The tree of life, the position of humans in this tree. We can see on the left the tree that Darwin himself wrote, in which he wanted; he hesitated to situate humans in relation to other species… And on the right Ernst Haeckel. In all the cases they were wrong in the details, they were right in the general view. And the general view, what genes have been able to show us, is how close, how within other species we are.
Skip to 15 minutes and 35 seconds So the interesting point now is to understand how is it that being so close, we are also so different.
The evolutionary clock of Humans
Are we really that different from our relatives, the bonobo, chimp, gorilla, orangutan?
Which is the living being closest to humans in terns of evolution? And what is the phylogeny?
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