When investigating cases of disappearances, we must first confirm several key facts. Investigators need to know when and where the missing person was last seen, and may need photographs to confirm what the missing person looks like and, of course, the name. Now imagine how the investigation would proceed without this information. About 150 years ago, when Charles Darwin published his theory of evolution, scientists began to accept the idea that humans share the same family tree as all other animals. Humans and apes are known to have shared a common ancestor, but what did these ancestors look like?
Darwin's theory necessarily leads to two conclusions: First, human beings are not the only descendants of our common ancestor. Somewhere in nature, there are at least animal species that are far closer to humans than other animals, and biologists call them "human sister species." Second, humans must have some ancient ancestor. Supposedly, if humans had 1 or more sister species, then we must have shared a common ancestor with them at some point in prehistory, a species evolutionary biologist calls the "Last Common Ancestor (LCA)." But most people know its non-scientific name better — the missing link.
Scientists have been searching for the "last common ancestor" for decades, but it hasn't been found. But many scientists believe they have discovered enough information to find this common ancestor easier and easier. They think they already know roughly when and where the "last common ancestor" lived, and can even reasonably speculate about what they look like and how they behave.
Figure 1: Chimpanzees
Before Darwin finally proposed the theory of evolution by natural selection, it was clear that humans had been classified as primates, although previous scientists did not consider this classification to have any evolutionary significance. Darwin was also initially reluctant to discuss human evolution directly, and he barely mentions this topic in his book On the Origin of Species. Darwin's colleague Thomas Henry Huxley was perhaps the first to attempt to confirm human origins with rational evolutionary thinking. In his 1863 book Evidence as to Man's Place in Nature, he said that anatomically human beings were very similar to gorillas and chimpanzees. Of the two species, 1 was certainly supposed to be a sister species for humans, though he didn't give a definitive answer either.
Huxley's ideas had a major impact on evolutionary biologists in the nineteenth and early twentieth centuries. Many people insist that chimpanzees or gorillas, or even both, are our sister species, but they go further. For these biologists, on the road to becoming humans, the apes seem to have become a stage representative of evolutionary history.
Figure 2: Gibbons may be distant relatives of humans
Small apes, such as gibbons, provide us with a window into the earliest ape ancestors. Large apes (such as gorillas, chimpanzees, and orangutans) also exhibit anatomical characteristics, and our ancestors broke away from other apes at some point in history and began to develop a unique human appearance. Gorillas and chimpanzees are not just our sister species, they are also very much like our "last common ancestors."
Tim White, an anthropologist at the University of California, Berkeley, said: "The post-Darwinian 'paradigm' adopts existing chimpanzees as stand-ins for the last common ancestor. This prompted concrete notions about the appearance and behavior of the 'last common ancestor.' Primates (especially monkeys) tend to have relatively small bodies, and they can run through the forest canopy with branches. But apes are unusual primates, most with strong bodies and long arms. They often swing around under the branches of the tree, rather than running along the top of the canopy, a form of movement called "brachiation."
Many early researchers claimed that the "last common ancestor" was an ape with a large body, long arms, and the ability to swing by branches. By the late 1960s, researchers were further fleshing out the image of the "last common ancestor." Anthropologist Sherwood Washburn points out that chimpanzees, especially gorillas, actually spend a lot of time on the forest floor moving on all fours.
Figure 3: Are chimpanzees our closest relatives?
The two apes walk in their characteristic way: they bend their fingers so that their weight is pressed against the knuckles. For Washburn, the "knuckle walking" style of "last common ancestor" is important. This behavior, he writes, can even be seen as a stepping stone for humans to achieve walking on two legs.
However, not everyone agrees. In fact, almost from the moment Huxley wrote his paper, a handful of scientists believed that the earliest and "last common ancestor" of mankind was certainly not a chimpanzee-like species. For example, just a decade after Huxley's book was published, biologist St George Mivart argues that humans share many common traits with monkeys and even lemurs.
Meanwhile, anatomist Frederic Wood Jones argues that humans and tarsiers seem to have more in common than chimpanzees or gorillas. Lemurs, tarsiers, and monkeys are all primates, but they have evolved independently from apes for tens of millions of years. How can anyone think that humans are close relatives to these groups? They have a simple and shocking explanation for this, with anatomist William Straus writing in the 1940s that humans are not "particularly evolved species."
Figure 4: A mountain gorilla
It seems absurd to think that we have highly developed brains as the best example of primate evolution. But human arms, hands, legs, and feet don't seem too special. Strauss writes: "It has been found that not only great apes, such as gorillas, chimpanzees, and orangutans, but also many more primitive animals have these traits. "The older the time it takes for species to separate, the more time those species accumulate their own molecular differences.
Strauss and other scientists have found that humans don't seem to have any proprietary features compared to apes that swing through trees. This makes sense, especially since humans were separated from other primates before apes evolved the ability to swing or "knuckle walk." Strauss couldn't be sure which species should be recognized as our sister species. But the "last common ancestor" may have been relatively small primates that were able to run along branches rather than sway under trees.
Nathan Young of the University of California, San Francisco, said the disagreement has persisted for decades. In fact, even into the 1980s, through anatomy, we still can't determine what kind of primate evolutionary tree humans originated from. However, after only a decade, this uncertainty disappeared. By the end of the 1990s, almost all evolutionary biologists were willing to accept the idea that chimpanzees and their close relatives, bonobos, together formed a sister species for humans. To understand the turning point in this story, we must look back at what has happened over the past few decades and look at a completely different branch of science.
Figure 5: Australopithecus lived 3 million years ago
In 1960, Nobel laureate chemist Linus Pauling was invited to write a paper in tribute to Szent-Gy?rgyi Albert, the discoverer of vitamin C. Together with his colleague Emile Zuckerkandl, he came up with a truly revolutionary idea: the molecular clock.
Jeffrey Schwartz, a physical anthropologist at the University of Pittsburgh in the United States, said: "This is a revival of the ideas put forward by bacteriologist George Nuttall in 1904, and if you compare serums, you can find the intimacy between species evolution. Their paper argues that molecules are constantly changing, and that the older the time it takes to separate species, the more time they have to accumulate their own molecular differences. ”
Pauling and Zukkander used the notion that some molecules accumulate small changes at a steady rate to analyze proteins in the blood of humans and gorillas. From the number of differences between the two groups of molecules, and estimates of the rate at which these differences accumulate, the researchers calculated that humans and gorillas shared the last common ancestor 11 million years ago.
Yet many anthropologists are unmoved. They believe that only fossils can tell us the times in which our common ancestors lived. It is said that many people describe the concept of Pauling and Zurkander as crazy, but molecular scientists have always stuck to their work. Decades later, they persuaded skeptics because new fossil discoveries provided conclusive evidence for their theories. All primate fossils, including great apes, were exposed by the 1960s. Among them, the name Ramapithecus or Sivapithecus looks a lot like the direct ancestors of humans.
Fig. 6: The jawbone of the rama ape
"Rama ancient apes were found in Pakistan about 14 to 16 million years ago," Schwartz said. They have thick enamel on their teeth, which is a distinctive feature we have found in humans and their immediate ancestors. "Chimpanzees and gorillas, by contrast, have only thinner enamel on their teeth.
In 1964, paleoanthropologists even speculated that rama apes walked on the ground in a very human-like manner and could use tools in preparing food. If 14 million years ago, Lamma was really the ancestor of humans, then gorillas and humans could not have had a common ancestor 11 million years ago, as Pauling and Zuckerkander put forward.
But these conclusions about rama apes are almost entirely derived from the study of ape teeth, and they can only be regarded as part of the study of ancient apes in the 1960s. In the early 1980s, more fossils of the ancient ape were unearthed, including fragments of the face. They show that apes look more like orangutans than humans. Paleontologists were shocked, but molecular scientists weren't surprised. Now they believe that there is a close relationship between humans, chimpanzees and gorillas, and that they shared a common ancestor 11 million years ago, while orangutans have a slightly more distant relationship with humans.
The scientists believe that orangutans from 14 million years ago may have looked very different from humans because they belonged to a pre-human lineage, but they may have looked a lot like orangutans. Schwartz said: "Molecular scientists may say: 'Look! We've always been right! "In the 1980s and 1990s, molecular science was built on such successes.
Figure 7: Western lowland gorillas
With breakthroughs in more complex molecular techniques, scientists can compare tiny details between apes at the genetic level and figure out which ones are most closely related to humans. Owen Lovejoy, an anthropologist at Kent State University in Ohio, said: "The gorillas were quite a reliable candidate, but in the end the chimpanzees won. ”
Eventually, scientists identified chimpanzees (and their close relative bonobos) as a sister species to humans in 1997, which seemed to herald the end of the "last common species" debate. Huxley's 1860 work encouraged many scientists to identify chimpanzee-like species as the "last common ancestor," while molecular scientists of the 1980s and 1990s seemed to prove his point. "People are starting to accept more generally, and eventually the common ancestor may be more like the chimpanzee narrative," Yang said.
That's not the only conclusion molecular scientists have worked so hard to reach. DNA studies have also shown the approximate date of a human-chimpanzee split, 6 million to 7 million years ago. This figure greatly narrows the range of the "last common ancestor" of the sola. The fossil record shows that about 20 million years ago, apes were widespread in Africa, Asia and Asia, and the world at this time was truly a "planet of apes". But by 7 million years ago, the great apes of Europe and Asia had disappeared. If chimpanzees and humans were to split at this time, the "last common ancestor" would have to live in Africa, in the same environment as modern chimpanzees.
By the early 2000s, some physical anthropologists even described African apes as chimpanzees, as time machines entered the earliest stages of human evolution. At this point, the story should be over, but it hasn't yet. Surprisingly, in the last 15 years, new theories have begun to emerge, and its views have begun to deviate from the idea that the "last common ancestor resembles chimpanzees" and approach the ideas put forward by Strauss and others in the 1940s.
Figure 8: Orangutans in Borneo (Pongo pygmaeus)
There are several factors that may explain recent reflections. Through dissection, it also allows us to learn more about chimpanzees and gorillas. For a while, many people called gorillas and chimpanzees (and bonobos) "knuckle walking" in exactly the same way. In 1999, Mike Dainton and Gabriele Macho of the University of Liverpool in the United Kingdom explored the idea more seriously. There are subtle differences in gorilla and chimpanzee wrist bone changes from adolescent development to adulthood, and Denton and Marjo conclude that the two species may have evolved joints that walk independently.
Over the next decade, other researchers reported similar findings. In 2009, Tracy Kivell of the University of Kent in the United Kingdom and Daniel Schmitt of Duke University at Durham in the United States argued that humans did not evolve from the "last common ancestor of knuckle walking.". Kiwell's paper received a lot of attention, probably because a few months after its publication, the most complete and important fossils for understanding human evolution were officially unveiled, which some believe found a huge loophole in the theory that the "last common ancestor resembled a chimpanzee."
Figure 9: Ardipithecus ramidus as envisioned by the artist
In late 2009, a research team that included Tim White and Owen Lovejoy published a collection of research papers describing the well-preserved "Adi" skeleton, the 4.4 million-year-old Ramida ape fossil, discovered by White and his colleagues in Ethiopia.
After careful analysis, White and Lovejoy firmly believe that "Adi" habitually walks on his legs. This is an important piece of evidence that "Adi" should be thought of as an early human, or Homo sapiens, who lived tens of thousands of years after the "last common ancestor", so it provides us with the best description of the "last common ancestor". This conclusion is important because in many ways, Adi's anatomy is different from all chimpanzees. It is likely that he was an early "knuckle walking" life, or an ape swinging through the woods.
"Adi" lives in the forest and definitely spends a lot of time on the ground and in the trees. But the dissection showed that, like a huge monkey, he was very well adapted to the surrounding environment of trees, while carefully moving with his feet. Unlike the feet of gorillas and chimpanzees, "Adi's" feet do not seem to be suitable for entanglement in branches to grasp.
Simply put, "Adi" looks very "primitive", while also indicating that the "last common ancestor" looks equally primitive.
Figure 10: The original appearance of "Adi"
Of course, the analysis of "Adi" is not without controversy. One explanation is that the various features found in the Adi anatomy have been found in gibbons, orangutans, chimpanzees, and gorillas, which evolved independently in these great apes. People are starting to question this new theory, With Kiwell saying, "I think they've gone a little too far, and their model means that there are many parallel evolutionary situations between all apes." I still think that comparative studies of chimpanzees and other African apes can provide a lot of insight into our own evolution. ”
Almécija of the George-Washington University agrees: "I believe chimpanzees can represent the 'last common ancestor' in some ways, such as body size, and even cognitive aspects." But his own research has also helped emphasize that chimpanzees may not simply live during the "last common ancestor" period.
In 2015, Olmosiga and colleagues published an analysis of ape hands highlighting changes in the length of their fingers since their separation from the "last common ancestor." Fossil evidence from early apes shows that, by appearance, human hands remain surprisingly primitive, despite the fact that we evolved forked thumbs after separating from the "last common ancestor."
Even biologists who study modern primates have found that the "last common ancestor" may not be a chimpanzee-like species.
Figure 11: Orangutans are our close relatives
In a 2013 study, Pavel Duda and Jan Zrzavy of the University of South Bohemia in the Czech Republic used known ape behaviors as well as ape evolutionary trees to try to estimate certain trait traits for first-time evolution. They argue that the "last common ancestor" sexual intercourse lasts longer than chimpanzees, and that the males in the "last common ancestor" need to spend more time caring for their offspring.
Strauss and several other anatomists agree that chimpanzees are the best model of the "last common ancestor," and mainstream public opinion has begun to step in. "There's been a shift in this community, and some people are starting to question the consensus that the last common ancestor was like chimpanzees,' Yang said. But that's not the end of the story, and there are still supporters of the "last common ancestor like chimpanzees" who fight back.
For example, Yang and his colleagues published a paper in 2015 saying that studies of ape shoulders showed that the "last common ancestor" may have many traits in common with chimpanzees and gorillas, suggesting that it may actually be a species that oscillates between forests. If it had been 10 years ago, such a conclusion would not have been controversial, but mainstream thinking is shifting from the idea that the last common ancestor was like a chimpanzee, which in fact now faces some criticism.
Of course, the debate will only end when fossils of the "last common ancestor" appear. But the search for those key fossils is no longer as straightforward as it once was. Over the past five years, some geneticists have begun to question whether they are correct in using molecular clocks to estimate the era in which the "last common ancestor" lived. It is quite possible that the "last common ancestor" actually lived 13 million years ago, not 7 million years ago. Apes were very powerful in Europe and Africa 13 million years ago, which means that in principle the "last common ancestor" may have lived there.
Figure 12: Dryopithecus of the forest envisioned by the artist
The idea could be supported by a 2015 analysis of forest apes, which lived in Africa and Europe about 12.5 million years ago. David Begun, an anthropologist at the University of Toronto in Canada, believes that forest apes may be early close relatives of gorillas, while the "last common ancestor" of humans and chimpanzees may have lived about 10 million years ago.
"It was almost impossible for the 'last common ancestor' to live in Europe," Began said. Although there is no direct evidence for his views, he still supports the idea that the "last common ancestor" lived in Africa. Of course, there are many researchers who hold completely different views. For example, Schwartz believes that orangutans are our sister species, not chimpanzees.
Schwartz first proposed the idea in the 1980s, saying that anthropologists acknowledge that molecular science is the ultimate arbiter of the shape of the ape family tree, not anatomy. Schwartz argues that DNA is unreliable evidence, and there is a lot of anatomical and behavioral evidence that should not be overlooked, and it can prove that the relationship between humans and orangutans is not simple. For example, both humans and orangutans have a thick layer of enamel on their teeth, and when in estrus, female orangutans (like females) do not show to males. Schwartz said: "Apart from humans, orangutans are the only mammals I know of that are not in heat. ”
Apparently, only a few researchers agree with Schwartz. But even putting his idea aside, it's clear that the "last common ancestor" has not yet reached a general consensus. Indeed, some researchers today have thoughtful consideration of the "last common ancestor," including their appearance and behavior. The problem is that other researchers also have good reasoning models that show that the "last common ancestor" is completely different in appearance and behavior.
This makes the research community feel torn between left and right. In principle, fossils of the "last common ancestor" could be the final argument, and they could be discovered this year. But because there are so many disagreements over the "last common ancestor," researchers may also interpret fossils differently. Olmosiga said: "This is a problem we may have. When we discover the 'last common ancestor', will we be able to recognize it? ”