The Framework of Human Evolution – Part Deux

Chapter 5 - The Systematic Context: From Linnaeus to Exons

Systematics involves the studying of the diversity of life by examining the evolutionary relationships between taxa, a term that can be used to describe any ‘level’ of classification, in an effort to reconstruct their unique evolutionary histories. Linnaeus invented the modern system of classification in 1758 based on anatomical characters, but predictably the field has grown rapidly over the last couple of centuries and now includes complex statistical formulas processed by computers as well as genetic evidence. There are essentially two schools of systematics: phenetics and cladistics. Pheneticists examine as many features as possible, the more features measured the more objective the study, and use statistical analysis to evaluate overall similarity and difference between taxa. Proponents of cladistics, or phylogenetic systematics, deem traits either ancestral or derived and reconstruct evolutionary relationships based on that determination. Hardliners on both sides of the argument claim superiority for their philosophy with cladists arguing that they study the only true evolutionary path and pheneticists claiming pure objectivity and a repeatable process.

Homologies

An essential component to both philosophies is accurately defining homologies, or homologues, referring to similarities between characteristics of organisms resulting from common ancestry. A very common example of a homology is the morphology of forelimbs in humans, dogs, birds and whales (illustrated above). Cladists claim that their analysis is more precise because they exclusively look at homologous traits whereas pheneticists statistically analyze as many traits as possible without speculating on what might be a derived character. Thus phenetics is not particularly helpful in determining common ancestry, but it provides a more complete estimation in similarity of characteristics between taxa.

Cladistics

Understanding several key terms are necessary to discuss cladistics. Primitive characters, those inherited from ancestors, are also known as plesiomorphies. Derived characters, those unique to the taxa being studied, are also known as apomorphies. A clade is defined by several taxa sharing a derived trait and, because that shared trait unites those taxa, the trait itself is dubbed synapomorphic. If a trait is only found in a single taxon, then that trait is dubbed autamorphic. These are all character/taxon-specific traits which are applied several taxa to group them into monophyletic, paraphyletic and polyphyletic groups. Cladists only accept monophyletic groups, or clades, which are groups where all taxa shared derived characters that emerge from a common ancestor. Clades are the only natural groups because they alone mirror true phylogeny.

More on cladistics-The book provides ample explanation about how cladistics is done in practice and Dr. Anemone drew an exercise on the board a few weeks ago in class illustrating this process.

Molecular Evidence

Since the 1980s this approach has been of increasing popularity and influence in evolutionary studies. This increase is due to the ever-increasing amount of molecular data (i.e. genome projects), the development of analysis techniques and the power of modern computers to conduct the analysis. In addition to DNA sequence-based research, analysis of proteins and DNA hybridization (comparison of genomes between species) is also common. Initially molecular research was seen as superior to morphology-based studies because genes were seen as the historical record of evolutionary change (provided we can learn the language it is written in) and that it was seen as immune to convergence by being selectively neutral (most changes in code not being selected for or against).

Molecular research has proved a very productive part of research on human evolution. It was this type of study that determined that humans evolved from our common ancestor of apes closer to 5 mya as opposed to 15-30 mya as previously thought (see figure below). Molecular systematics is, however, not seen as the sole solution to understanding hominin evolution anymore. The specifics of mutation and the varying degree of gene susceptibility according to type have proved elusive to researchers. The view of a single molecular clock has been replaced by the idea that individual gene types each having their own clock ticking at different rates. Both molecular and morphological analyses have a place in reconstructing the path of human evolution and are best used in conjunction with one another rather than competing against one another in search of legitimacy.



Questions:

-Which method (cladistics or phenetics), solely based off of what we have read up to this point, offers the best technique to help explain human evolution? Why?

-Molecular research has a very real impact on the field. In the practice of gorilla conservation molecular studies are employed to estimate similarity between groups and greatly influences conservation practices. Based on the evidence presented in this chapter, how much of weight should be given to molecular analyses be allotted given that it seems the more we think we know the less we actually do?


Chapter 6 – Human Evolution in Comparative Perspective

It is essential that humans are viewed in comparison to other animals in order to discern what is unique to us as a species. This chapter focuses on this type of inquiry and its two basic elements: that we share synapomorphies with other primates and rules govern attributes such as energy, size, and shape in biological systems (126).

Primate Heritage

When examining primates, or any other taxa of animals, it is important to remember that each group inherits a basic set of anatomical and behavioral characteristics by which that group is recognized, but that each species is also unique due to its specific (no pun intended) evolutionary path. Primates come in four main types: prosimians, New World monkeys (NWM), Old World monkeys (OWM) and hominoids (apes and humans). Modern classification places tarsiers closer to anthropoids than prosimians, but this aside the general view of primate taxonomy has been rather consistent. Primates are defined by common characteristics, such as grasping hands, hind-limb dominated locomotion, stereoscopic vision, encephalization, and relatively slow reproductive rate in comparison to other mammals. Three main theories postulating the reasons behind the success of these characteristics, and thus primates in general, exist. The arboreal theory, put forth in the early 1900s, was the first to emerge. It claims that life in the trees emphasized the need for nails instead of claws, grasping hands and increase reliance on vision for locomotion in branches. The visual-predation hypothesis followed claiming that the decreased reliance on olfaction and increased reliance on vision while searching and capturing prey allowed for the success of primate characteristics associated with this activity (close-set eyes, grasping extremities, reduced claws, etc). The third theory is that primate radiation was in sync with angiosperm (flowering plants) radiation. The ability to locate and feed on flowering plants would again allow for the success of organisms having primate characteristics. There are problems, however, with all of these theories. Many arboreal animals do not possess primate characteristics and are very successful, early primates were insectivores and research shows that insectivores rely more on hearing than sight, and the timing of angiosperm and primate radiation does not exactly match up. Most likely, it is a combination of all of these along with others that we have not thought of yet that explain the success of primates. Aside from the specifics of the tale of primate success, a constant pattern of increased brain size, intelligence, behavioral flexibility and social complexity is clearly visible in primate evolution. Humans should be seen as merely continuing this trend, rather than traits specific to humans being viewed as novel.

The Comparative Perspective

As stated in the introduction for this chapter, uniqueness can only be recognized when comparing one organism to another. A less anthropocentric perspective has allowed for science to progress successfully in this area. From this area we have been able to discover similiarities along with illustrating uniqueness, but it has been proven that both plesiamorphies and anapomorphies look similar when using this technique. Further progress has been made through models that take this into account.

Bodies, Size, and Shape

The size and shape of bodies is directly related to an organism’s adaptation to its environment. Climate, for instance, is the underlying factor for surface area/mass ratio as stated in Bergmann and Allen’s rules. Variation in size between populations reflects the evolution of solutions for balancing heat production and dissipation at different latitudes. Humans living in colder climates tend to have larger mass and less surface area when compared to populations living in more temperate climates. Even pygmies, sometimes seen as anomalies to this trend, comply as they are typically found in humid forests where it would be beneficial to reduce one’s volume (decrease size) to lessen the amount of heat generated by the body. Some population migration patterns, like that of the Pecos Pueblo Amerindians, complicate this view because of their slow evolution away from large body width. With respect to human origins, many anthropologists believe that AMH evolved in Africa and radiated out of the continent. This idea that tall, long-limbed people entered lands more conducive to a stouter, robust build is called the climatic hypothesis. A competing hypothesis is the mobility hypothesis that holds early modern Europeans outcompeted Neanderthals because of their long stride length allowing for covering more territory for foraging.

Recent trends in human evolution include the loss of robusticity, decreased brain size, size of jaws and teeth and overall stature. The loss of robusticity is explained by the increase in technology, lessening the reliance on brute strength. Nutritional stress is viewed as the primary factor in the decrease stature in humans. This type of stress has been directly related to the increase in size of human populations that created a shortage of resources. Body-size reduction has also occurred in many other animal populations during the same period as humans and is attributed to the increase in global temperature at the end of the Pleistocene.

Bodies, Brains, and Energy

The size of brain and bodies, along with the energy requirements associated with that size, greatly impact the individual life-history of animals. Reproduction requires so much investment that animals have come up with strategies that maximize their Darwinian success. The r-K model offers insight into this matter. It states that the growth of a population is determined by two things: its fecundity and the availability of resources. “r-selected” species have high reproductive and high mortality rates. This maximizes reproductive success in unstable, unpredictable environments. “K-selected” species, on the other hand, have few high-quality offspring indicative of living among limited resources resulting in intergroup competition, thus more parental investment. Overall, primates are large-bodied mammals and this comes with several benefits. For hominins this include a broad diet, large day and home-ranges, high mobility, increased sociality, and enhanced encephalization becoming energetically possible.

Questions:

-How will humans evolve next given current environmental pressures keeping in mind the contraints and patterns discussed?

Chapter 7 – Reconstructing Behavior

Bodies, Behavior and Social Structure

The importance of sociality in primates cannot be understated and studying social organization is a vital part to understanding early hominin populations. Since none of us were around for early hominin evolution, we have to infer about the social organization of early hominins by examining living primates. A species’ social organization is comprised of the size, composition and activity of a group (164). This chapter examines the causes associated with living in groups as well as the way individuals adapt behaviorally and anatomically to social structures.

There is a great variety of social organization found just within the apes. Gibbons are monogamous, orangutans practice “exploded” unimale polygyny, gorillas practice a more tight-knit form of unimale polygyny and chimps practice multimale polygyny. Each species must be examined within their specific evolutionary contexts in order to determine the cause of their social organization, but there are overarching explanations out there. Richard Wrangham claims that selection pressure on female behavior determine the effect of ecology on groups (168). In this view, if resources are patchy then females tend to forage alone and where resources are dense matrilocal, or female kin-bonded, social organization evolves. The high energetic cost of reproduction and postnatal care is why females influence social organization more than males. Sexual dimorphism can also be explained by social organization because the prevalence of male-male competition would select for larger males. Some researchers, like Robert Martin, suggest that science should look towards the possibility that the shrinking of females, instead of the increase in the size of males, as an explanation for sexual dimorphism

Non-human Models of Early Hominin Behavior

The two major goals of using non-human primates as analogues for early hominins are to study what is seen as “the starting point for human evolution” and gain an understanding of primate socioecology in an effort to reconstruct the behavior of extinct hominins (174). This is practiced via three different models. The earliest of the three techniques involves the selection of a specific species that shares characteristics with hominins and try to transfer lessons of their behavior to hominins. Many species of animals have been used in this way for a variety of reasons, including baboons, chimps, lions and even marine mammals. An alternative model is the phylogenetic comparison which examines shared behavioral characteristics of humans and other primates in order to determine what behaviors are derived or ancestral. The behavioral ecology model is the newest of the three techniques and involves the study of how environment can effect social organization, and therefore behavior as well.

Jaws and Teeth

Jaws and teeth are extremely important to reconstructing evolutionary paths of hominin species if for no other reason than the fact that they are plentiful in the fossil record (due to their density) relative to all other fossil remains. There are four aspects of hominoid dentition that tell us much about the behavior and life-history of our ancestors. First, the structure of the jaw and the morphology of teeth illustrate an increase in both brain size and a more generalized diet associated with human evolution. Second, eruption patters differ in apes and humans, thus indicating the degree of relatedness between extinct species and AMHs by their age of tooth eruption. Third, enamel thickness increased throughout human evolution with humans having much thicker enamel relative to apes. Finally, toothwear patterns shed light on the dietary patterns of their former owners.

Questions:

-Which should be given more weight: comparative analysis between taxa that share similar anatomical characteristics (humans and apes) or those that share(d) similar ecological conditions (scavenging humans and hyenas)?

-How might life-history characteristics among current human populations change provided that our lives remain relatively consistent with the present over the next couple hundred of thousand years?