What is evolution?
In its original sense, evolution meant “unrolling”, as if a papyrus scroll were being unrolled to reveal its contents. We may talk about the “evolution” of many things, from an individual’s lifetime to the evolution of the universe. In the most general sense, evolution means “change”.
Biologists are very specific about the kinds of processes that qualify as “evolution” in the biological sense. Biological evolution is genetic change in a population over time. Populations and individuals change in many ways, but only some changes are evolution.
Here’s a list of seven things about evolution. It’s not comprehensive but it hits on several important issues that help to understand how evolutionary biologists think about the process of evolutionary change.
Evolution is change in a population. Individuals change during their lifetimes, even day to day. Those changes are not biological evolution, although they may be products of evolution in past populations. Likewise, a forest may change over time, as some kinds of trees proliferate and others disappear. Those changes in community structure are not themselves biological evolution, although they may influence the evolution of the populations of trees composing the forest.
Evolution is genetic change. Many kinds of phenotypic changes don’t involve evolution. For example, many human populations have markedly increased in lifespan during the last 100 years, mostly as a result of improvements in nutrition and reductions in disease. Those changes are important and highly visible, but they are not biological evolution. Physical characteristics and behaviors can only evolve if they have some genetic contribution to their variation in the population — that is, if they are heritable.
Many kinds of genetic changes are important to evolution.Mutations happen when a DNA sequence is not replicated perfectly. A sequence may undergo a mutation to a single nucleotide, small sequences of nucleotides can be inserted or deleted, large parts of chromosomes can be duplicated or transposed into other chromosomes. Some plant populations have undergone duplications or triplications of their entire genomes. These patterns of genetic change can have a wide range of effects on the physical form and behavior of organisms, or may have no effects at all. But all of them follow the same mathematical principles as they change in frequency within populations.
Evolution can be non-random. Populations of organisms cannot grow in numbers indefinitely, so that individuals that successfully reproduce will have their genes increase in proportion over time. Among the genes carried by such successful individuals may be some that actually cause them to survive or reproduce, because they fit the environment better. The survival and proliferation of such genes is not a matter of chance; it is a result of their value in the environment. This process is called natural selection, and it is the reason why populations come to have forms and behaviors that are well-suited to their environments.
Evolution can be random, too. Many genetic changes are invisible and make no difference to the organisms. Many changes that do make a noticeable difference to the organisms’ form or behavior nevertheless still do not change the chance of reproducing. Even individuals with the best genes still have a strong random component to their reproduction, and in sexual organisms genes assort randomly into sperm and egg cells. As a result, even when an individual has a beneficial gene that increases the chance of reproducing, that valuable gene still is very likely to disappear quickly after it first appears in the population. Genetic drift is strongest when populations are small or genes rare, but it is there all the time. Random chance has a continual role in evolutionary change.
Populations evolve all the time. No population can stay static for long. Reproduction is not uniform, and no organism replicates DNA perfectly. The genome of the simplest bacterium has thousands of nucleotides, ours has billions. Keeping these sequences constant, generation after generation, is a task no population has ever managed to do. Genetic variation is constantly introduced into populations by mutation and immigration, rare genetic variations are constantly disappearing when individuals who carry them don’t pass them on, and occasionally rare genes become common — whether by natural selection or genetic drift. If a population’s physical form remains the same for a long time, we have a good reason to suspect that natural selection is working to oppose random changes.
Evolutionary theory has changed a lot since Darwin’s day.Charles Darwin recognized several key insights about biological evolution, including the process of natural selection, the tree-like pattern of relationships among species, and the potential for significant changes when processes act through small, incremental steps across geological timescales. But we know a lot more now than Darwin knew. We understand the molecular basis of genetic changes, and many of the ways that the features of organisms can be affected by genetic and environmental change. We have learned much about the limits of evolution, the alternative patterns of change caused by environments, and the importance of randomness. We now know much about the changing pace of evolution, seeing it as a dynamic process that can happen in fits and starts.
Evolution is the most powerful idea in biology, organizing our knowledge about the history and diversity of life. We understand our own origins using the same tools that we use for organisms across the tree of life, from the simplest bacteria to the largest whales.
Karen Hardy and colleagues (2013) have a brief paper in a recent issue of Antiquity putting into context their recent finding about possible medicinal plant use by Neandertals. In 2012, this team of authors reported on their examination of the dental calculus of the El Sidrón Neandertals. They found some evidence for plant food consumption, in line with results from other Neandertal sites. But additionally they found chemical traces of other interesting things:
Evidence of oil shale or bitumen. Bitumen was used by Neandertals at other sites as an adhesive for hafting stone points onto wooden spears or handles. That evidence came from the stone points themselves, so it is possible that bitumen was used more widely as an adhesive or preservative in contexts that do not persist as long in the archaeological record.
Alkyl phenols and polynuclear aromatic hydrocarbons consistent with exposure to wood smoke or smoked food.
Chemical residues consistent with consumption of yarrow and chamomile, including bitter-tasting and appetite-suppressing compounds.
A very low level of proteins and absence of lipid components suggested that the diet of these individuals was protein-poor during the time they were forming calculus.
They reinforced conclusions about cooking plant foods by Neandertals, based on both the chemical evidence and the examination of starch granules embedded in the calculus:
Using mass spectrometry, we have identified the ingestion of cooked carbohydrates in the calculus of two adults, one adult in particular having apparently eaten several different carbohydrate-rich foods. The evidence for cooked carbohydrates is confirmed both by the cracked/roasted starch granules observed microscopically and the molecular evidence for cooking and exposure to wood smoke or smoked food in the form of methyl esters, phenols, and polynuclear aromatic hydrocarbons (notably pyrene and fluoranthene) found in the dental calculus.
The more intriguing observation was the yarrow and chamomile consumption. Hardy and colleagues considered it likely that these plants were used for medicinal purposes by the Neandertals. They discuss the botanical qualities of these plants briefly in their current paper (2013):
Yarrow is a flowering plant in the Asteraceae family, common across temperate regions. It was used as a vegetable in the Middle Ages, notably as a component of soup, but has an extended history of medicinal use, in particular as an astringent (Chandler et al. 1982). Camomile tea is well-known today as an aid for stomach complaints and nervousness, though there is little record of it as a food. Bioactive constituents are linked to antimicrobial and anti-inflammatory properties (McKay & Blumberg 2006), while its ability to assist with general anxiety disorder has been demonstrated (Jay et al. 2009).
Laura Buck and Chris Stringer (2013) suggested an alternative explanation for the yarrow and chamomile. They note that arctic peoples often eat the stomach contents of animals they eat, which comprises one of the major sources of plant foods in their diet. In such cases, the plants are not necessarily those most palatable or digestible by humans.
We are not, of course, proposing that Neanderthals would not have eaten plant foods, nor are we discounting the possibility of Neanderthal self-medication. However we suggest that, given the evidence for widespread consumption of stomach contents in recent human groups, and the likely benefits of a rich source of vitamin C and carbohydrates (to say nothing of the possible cultural or social reasons for chyme consumption) this behaviour should be taken into account as a possible source of plant foods, including ‘medicinal’ ones, in the archaeological and fossil record.
Hardy and colleagues (2013) do not directly react to this argument (which may have emerged after submission of the article), but they do note that the El Sidrón Neandertals lived during a time of relatively mild climate when plant foods would have been easily available in the immediate surroundings. The evidence for cooked starch granules and paucity of protein further suggest that the plants were selected and used by the Neandertals rather than opportunistically consumed as part of herbivore stomach contents.
But these possibilities are not mutually exclusive, either. From my point of view, one of the most likely ways that Neandertals may have accomplished the gelatinization of starches is by cooking grains and other plants inside of animal bladders, including the stomach.
We may also consider that both yarrow and chamomile are used for dying fabrics, so it is not impossible that the Neandertals were processing them as pigments by chewing them. They are already known to have used red earth pigments and black manganese pigments.
Hardy and colleagues finish their 2013 paper by considering what self-medication would mean for our understanding of Neandertal behavior:
Though all primates (and other animals) have varying levels of enzymes which make us more or less tolerant of certain toxins, there are plants which are poisonous to all; in order to survive, hominins needed to know which plants not to eat and how and when to eat those plants they selected. The use of edible bitter tasting plants by the Neanderthals of El Sidrón suggests their knowledge was sufficiently refined to use plants with confidence even when their bitter taste warned of potential toxicity. This demonstrates that their knowledge of plants was at least equal to today’s higher primates; with their additional linguistic and technological abilities it may have been far more elaborate. Rather than contradicting the extensive evidence for consumption of meat, the evidence for the use of plants adds a rich new dimension to our developing knowledge of Neanderthal life. We can never know for sure why yarrow and camomile were ingested at El Sidrón, but we propose that the evidence for self-medication offers the most convincing behavioural context.
An aside: I know there are anthropologists who will take in this evidence of interesting Neandertal behavior and argue they had a specific “module” in their mind that facilitated naturalistic knowledge, while simultaneously arguing they lacked some crucial “module” enabling modern human social behavior.
That’s special pleading. We do not need to hypothesize that Neandertals had a multicameral mind to explain that their cultures were different from ours. Cultures do not weigh every kind of activity or knowledge equally or enable them to be learned equally easily.
Buck, L. T., & Stringer, C. B. (2013). Having the stomach for it: a contribution to Neanderthal diets?. Quaternary Science Reviews. (in press)doi:10.1016/j.quascirev.2013.09.003
Hardy, K., Buckley, S., & Huffman, M. (2013). Neanderthal self-medication in context. Antiquity 87 (2013): 873–878. URL: http://antiquity.ac.uk/ant/087/ant0870873.htm
Hardy, K., Buckley, S., Collins, M. J., Estalrrich, A., Brothwell, D., Copeland, L., … & Rosas, A. (2012). Neanderthal medics? Evidence for food, cooking, and medicinal plants entrapped in dental calculus. Naturwissenschaften, 99(8), 617-626.