Transforming our thinking about transitional forms. Problem concepts in evolution part I: purpose and design. Problem concepts in evolution part II: cause and chance. Misconceptions about the evolution of complexity. Scott EC. Countering creationism with drive-by science.
Gen Anthro. Accept it: talk about evolution needs to evolve. Sci News. Download references. We thank Louise S. Mead and Glenn Branch for helpful discussion and suggestions and Steven Newton for preparing the figures.
National Center for Science Education, P. You can also search for this author in PubMed Google Scholar. Correspondence to William Eric Meikle. Reprints and Permissions.
Meikle, W. Why Are There Still Monkeys?. Evo Edu Outreach 3, — Download citation. Published : 16 October Issue Date : December Anyone you share the following link with will be able to read this content:. Many scientists are people of faith who see opportunities for respectful dialogue about the relationship between religion and science.
Some people consider science and faith as two separate areas of human understanding that enrich their lives in different ways. This Museum encourages visitors to explore new scientific findings and decide how these findings complement their ideas about the natural world. In science, gaps in knowledge are the driving force behind the ongoing study of the natural world and how it arose.
The science of human origins is a vibrant field in which new discoveries continually add to our understanding of how we became human. You can learn about some of the most recent findings in this exhibit.
Societies worldwide express their beliefs through a wide diversity of stories about how humans came into being. These stories reflect the universal curiosity people have about our origins. For millennia, they have played a vital role in helping people develop an identity and an understanding of themselves as well as of their community. This exhibit presents research and findings based on scientific methods that are distinct from these stories.
Skip to main content. How does evolution work? What do scientists mean when they call evolution a theory? How does evolution explain complex organisms like humans? How are humans and monkeys related? Did humans evolve in a straight line, one species after another? How do scientists know the age of fossils?
How do scientists know what past climates were like? However, there are a number of plausible explanations. First, we must distinguish between 1 the processes that led to brain size increases as a result of relaxed selection against larger brains and 2 those that actively favored increases in brain size. Relaxed selection, the elimination or substantial weakening of a source of natural selection, can be an important factor leading to change in natural populations Lahti et al.
For instance, suppose zebras are under threat from predation by lions. If, for some reason, lions disappeared, then that source of selection would be relaxed. The relaxation of this selective pressure might influence the evolution of zebras along a different trajectory than that constrained by lion predation.
Similarly, relaxed selection may have permitted evolutionary change in the human lineage. Dietary changes are often presented as explanations for the increase in brain size. A notable example is an increase in meat eating.
Our ancestors started eating considerable quantities of meat about 2. Meat not only provides animal protein, but also creatine, an important molecule for long-term energy storage in the brain Babbitt et al.
The switch to meat and other changes probably made it easier for our ancestors to evolve larger brains because such changes relaxed selective constraints on energetic requirements.
By contrast, the lack of extensive meat eating by the ancestors of chimps may be one reason why they did not evolve larger brains. Relaxed selection, however, cannot be the whole story behind brain evolution. Scientists must hypothesize that such an advantage occurred because brains are costly. Our large brains also make birthing difficult; indeed, the human gestation period is reduced and brain growth continues throughout infancy, owing to constraints by the mother's pelvic morphology.
Such large costs require that larger brains provided some large advantage or increase in reproductive success. One popular explanation is that some feature of the social environment of our ancestors provided a selective advantage to individuals with larger brains. Strong support for this explanation is that social group size of different primates is tightly correlated with brain size Dunbar, Perhaps larger brains evolved because individuals with refined cognitive skills were better able to keep track of information in the group and to use that information to their advantage Dunbar, Another possibility is sexual selection: perhaps males and females preferred mates that were more skilled.
Geoffrey Miller proposed that such sexual selection may be responsible for such traits as language, music, and the arts. If we assume that DNA sequence differences accumulate as a function of time, then, because human and chimp lineages have been diverging for the same amount of evolutionary time, we would expect that the same amount of evolutionary change has occurred in each lineage Scenario III.
Such a pattern has been seen in some DNA sequences. For example, Chen and Li found that parts of DNA in-between genes evolved at similar rates in humans and in chimps. There is a third possibility: more evolution along the chimp lineage Scenario II. Biologists are now increasingly able to detect the signature of adaptive evolution acting on particular genes Johnson, This information can be used to address whether adaptive evolution has been more prevalent in certain lineages, such as the human one, as compared with others.
Considering just the 14, genes that encode proteins, of these genes show the signature of having undergone adaptive evolution in the lineage leading from the human—chimp common ancestor to chimps. By contrast, only underwent adaptive evolution in the lineage leading from the human—chimp common ancestor to humans Bakewell et al. On the basis of this information, we come to the potentially counterintuitive conclusion that more adaptive evolution has occurred within the chimp lineage than in the human lineage.
See Futuyma for an explanation regarding why natural selection is more efficient in larger populations. Many of the adaptive evolutionary changes that have occurred since humans and chimps split have nothing to do with the brain, but instead involve changes in the immune system, reproductive functions, and other biological processes. In those cases above , faster evolution occurred in the chimp lineage. When we look at the changes that have occurred for genes that have major roles in the brain, a different picture emerges.
One of these genes is FOXP2. Humans who carry mutations in FOXP2 have difficulty in speech, especially in articulating complex words. These people also have other difficulties in coordinating movements and show clear differences in various brain scans when compared with the general population reviewed in Johnson, ; Taylor, FOXP2 shows a highly unusual pattern; of the three genetic differences that alter the amino acids in its protein, two differences have evolved since the human and chimp lineages diverged.
Such a pattern is highly statistically unlikely to occur by chance. These and other data strongly suggest that natural selection has operated on this gene that affects language and brain activity since the lineages diverged, and possibly within the past , years Johnson, ; Taylor, Two other genes, ASPM and Microcephalin, which influence brain size, also show a clear pattern that they have evolved via natural selection within the human lineage Johnson, ; Taylor, No genes such as these that affect brains have been found to show adaptive evolution occurring on the chimp lineage since it split from the human lineage.
Of course, genetic changes that alter the structure of proteins are not the only type of genetic changes that occur. Also of importance are regulatory mutations, which lead to changes in when, where, and how much protein is put into a particular cell. Regulatory mutations alter gene expression, not gene structure.
In , King and Wilson predicted that regulatory changes could have a much more profound effect on the organism than changes in the building-block proteins themselves. In the past few years, biologists have confirmed this prediction: regulatory mutations can and do have major effects. What is known about regulatory mutations within the human and chimp lineages? In one of the first studies to examine relative rates of gene expression in humans, chimps, and macaques a monkey , Enard et al.
Since the publication of Enard's study, more studies on differences in gene regulation between humans and chimps have been done. Further work by Haygood et al. More recently, Greg Wray and his colleagues at Duke University have been comparing how glucose is allocated differentially to the brains and muscles of different primates as measured by the numbers and activities of a set of glucose transporter proteins; see Zimmer, , presumably in response to diet type.
Wray's group found that the relative numbers of glucose transporters for the brain has gone up in the human lineage, whereas the number of glucose transporters for body muscles has decreased. This is in direct contrast to observations made in chimps in which the muscle transporters are abundant and the brain transporters are comparatively fewer in number.
This study, combined with other work recently published by the Wray lab on the phosphocreatine cycle Pfefferle et al. In summary, although it appears that overall evolutionary rates in the human and chimp lineages have been roughly equivalent since the time of the human—chimp split, certain types of evolutionary changes have occurred predominantly in the human lineage.
These changes have frequently occurred at the level of gene regulation and appear to have occurred in biological features that involve the human brain. Answer: For most physical traits, especially brain size, the common ancestor looked more like a chimp than a human. Consider all of the members of the superfamily Hominoidea hominoids — the group that includes all the apes that are living today and all of the tailless primates that have lived in the past.
Among the homonoids, humans and our recent fossil ancestors are the most flat-faced orthognathic , large-brained, and small-toothed. Modern chimps are much more like fossil apes, gorillas, orangutans, and gibbons than are humans. Parsimony the task of finding the simplest explanation that fits the data tells us that modern humans have uniquely derived traits not shared with any other hominoids — and these traits set us apart from chimps. Given the similarities between modern chimps and other fossil and nonhuman apes, scientists hypothesize that modern chimps are much more similar to the human—chimp common ancestor than are modern humans.
Learn More Origins of Humankind. A society's culture consists of its accumulated learned behavior. Human culture is based at least partly on social living and language, although the ability of a species to invent and use language and engage in complex social behaviors has a biological basis.
Some scientists hypothesize that language developed as a means of establishing lasting social relationships. Even a form of communication as casual as gossip provides an ingenious social tool: Suddenly, we become aware of crucial information that we never would have known otherwise. We know who needs a favor; who's available; who's already taken; and who's looking for someone -- information that, from an evolutionary perspective, can mean the difference between failure and success.
So, it is certainly possible that evolutionary forces have influenced the development of human capacities for social interaction and the development of culture. While scientists tend to agree about the general role of evolution in culture, there is still great disagreement about its specific contributions. There is great debate about how we are related to Neanderthals, close hominid relatives who coexisted with our species from more than , years ago to about 28, years ago.
Some data suggest that when anatomically modern humans dispersed into areas beyond Africa, they did so in small bands, across many different regions. As they did so, according to this hypothesis, humans merged with and interbred with Neanderthals, meaning that there is a little Neanderthal in all modern Europeans.
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