Structural Biochemistry/Evolution of Humans

Evolutions of Humans

Humans come from the genus Homo and species sapien. Through the widely accepted classification system of Kingdom, phylum, class, order, family, genus and species. Humans all come from vertebrates which have a great line of divergence, some of the vertebrates are Lancelets, urochordtaes, craniates, tunicates, ray finned fish, lobed finned fish and from the lobed finned fish emegered the terrestrial vertebrates and mammals, and out of the mammals in a group called primates came humans. Humans are considered to be members of apes and come from the old world monkeys as opposed to new world monkeys. Old world monkeys have tails but are weak and unlike new world monkeys are unable to use their tails to do tasks such as hang on trees. Many primates share derived characteristics. They have hands and feet which allows them to hold things and reach for them. They have opposable thumbs useful for grabbing and holding, they are developed and conditioned culturally and socially. They have vision different from other animals which allows them to perceive depth .They also have larger brains. From the old world monkeys hominoids diverged around 20-25 million years ago. Humans are in this group and are called bipedal hominoids. The human species we know today, Homo sapiens are about 160,000 years old and are relatively young for an Earth that existed about 4.6 billion years ago. Hominoids have derived characteristics as well. They shared similar derived characteristics such as hairlessness, have larger brains, stand upright are able to communicate through language, shorter jaws and are able to create tools and instruments. It is correlated that hominoids came from Africa about 6 million years ago. The earliest of hominoids had small brains and walked upright.

Hominin Evolution: Approximately 6-7 mya, the last common ancestor between humans and chimpanzees diverged into at least two separate lineages. Hominins refer to the group of organisms in the lineage that gave rise to humans. The candidates for earliest hominin include Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus kadabba, and Ardipithecus ramidus (Klein 2009).

As its name suggests, Sahelanthropus tchadensis was found in Toros Menalla, Chad (6-7 mya). The fossil that belongs to this species was nicknamed, “Toumai,” and consisted of an almost complete cranium. Based on the anterior position of the foramen magnum, it is likely that Sahelanthropus tchadensis was a bipedal organism. However, its brain size of 360-370 cc is far smaller than that of the average person. Because the Toumai fossil had no postcranial features and the skull was not completely intact, there is debate as to whether Sahelanthropus tchadensis was an ape and not an ancestor of modern humans.

Orrorin tugenensis was found in the Tugen hills in Kenya and is dated to approximately 6 mya. The curved phalanges on this species indicate that it was skilled at tree-climbing. However, the long neck of the femur and the location of the obturator externus groove support bipedalism.

The last 2 candidates for earliest hominin belong to the genera Ardipithecus. Most of the specimens for Ardipithecus kadabba are isolated teeth that date 5.2-5.8 mya. In contrast, there is an almost complete skeleton specimen for Ardipithecus ramidus, which dates to 4.4 mya. Ardipithecus ramidus had traits in between those of apes and humans. The anterior position of the foramen magnum suggests that Ardipithecus ramidus was bipedal. In addition, unlike some apes, there was no evidence of knuckle-walking adaptations. However, while its legs were adapted for bipedalism, Ardipithecus ramidus also had an opposable big toe and partially grasping feet to facilitate an arboreal lifestyle, both of which are traits associated with apes. Therefore, based on current evidence, it is unclear whether or not any of the proposed candidates are hominins that eventually gave rise to humans or if they merely represent ape lineages that went extinct.

Australopithecines: Australopithecines represent a subfamily that has enough fossil data to support them as the oldest known hominins. They existed from 1.2-4.4 mya and had human bipedalism and other traits as well as ape-like traits, including a smaller brain size and adaptations for climbing trees. Australopithecines can be divided into 4 genera: Ardipithecus (ramidus), Australopithecus, Kanyanthropus, and Paranthropus. Ardipithecus ramidus is sometimes included in the Australopithecine subfamily because its placement in the fossil record remains unclear.

One of the earliest Australopithecines was Australopithecus anamnesis dated 3.9-4.2 mya. While foot bones suggest bipedalism, curved phalanges and other hand traits indicate an ability to knuckle walk and climb trees. Other hominin features include large molars and thick enamel on its dentition.

Australopithecus afarensis is one of the better documented Australopithecines. Some of the more famous finds include “Lucy,” “The First Family,” and “Dikika Child.” Lucy was an almost complete skeleton found in 1974 that dates to 3.2 mya. The First Family includes 13 individuals found at the same site in 1975. Finally, the Dikika Child is a fossil of a 3 year old with a brain size of 300 cc, which represented about 75 % of the adult brain size. This indicates that Australopithecus afarensis matured more slowly than chimpanzees. In general, Australopithecus afarensis shared features with both apes and humans. Like humans, Australopithecus afarensis had a longitudinal arch in its foot and a broad, basin-shaped pelvis that facilitated bipedalism. However, they also had small brains (400 cc) and had a diastema in between that of chimpanzees and humans (who lack it completely).

Bipedalism vs. Brain Size: The finding of hominin fossils in Africa demonstrate that bipedalism came earlier in evolution than an increase in brain size comparable to that of modern humans. Therefore, it is important to understand why bipedalism was chosen as the mode of locomotion in human evolution. There are currently many theories as to why hominins became bipedal.

One of the theories on why hominins became bipedal was that it was easier to carry tools, foods, children, and other resources from one location to another. Another idea states that bipedalism allowed people to see over tall grass so that they could avoid predators and search for prey. Finally, it is possible that it was more energetically efficient to walk on two feet. It is possible that a combination of all of these reasons as well as other reasons led humans to become bipedal. However, whatever the reason may be, there are certain drawbacks to bipedalism that had to be considered. For example, the restructuring of the pelvis that allows for bipedalism also leads to difficult births. Furthermore, since the weight increases as you move down the spine, bipedalism also causes lower back pains. Therefore, the benefits of bipedalism must have outweighed the costs in order for hominins to adapt to this type of locomotion.

o Homo

 Homo habilis • Lived anywhere from the range of 2.4 – 1.6 mya, they used tools to hunt.

 Homo ergaster • Lived about 1.9 to 1.6 million years ago. They were able to walk on two, and had large brains.

 Homo erectus • Came from Africa, existed around 1.8 mya. They were the first hominoid group to leave Africa .

 Homo neanderthalensis

•Neandertal and Anatomically Modern Human Co-existence: The relation, if any, between Neandertals and Homo sapiens remains unclear. While some have suggested that Neandertals are members of the genus Homo, others contend that the hominid species belongs to a different genus. The debate centers on the level of interaction between Neandertals and anatomically modern humans (AMH), with particular emphasis to their co-habitation of Europe, which may have lasted as little as 5,000 years. Based on the current fossil evidence, it is supported that AMH had spread across Europe by 38 kya. In addition, there is fossil data that suggests that Neandertals may have existed as recently as 34-38 kya (Joris et al., 2011; Doronichev et al., 2011). Therefore, there is evidence of overlap both in time and space between the two groups. However, what is less clear is the level of interaction and interbreeding that took place. How much of the human genome comes from Neandertal genes and what role did humans have in Neandertal extinction? In addition, if humans did cause the Neandertals to go extinct, what evolutionarily favorable traits enabled them to outcompete a hunter species with larger brains and an anatomy built to combat the cold climate of the time.

Hypotheses on Neandertal Extinction: As stated above, the fossil record shows Neandertal extinction around 34-38 kya. However, fossil evidence also reveals that Neandertals may have lived as long ago as 130 kya. So, soon after their encounter with AMH, they went extinct. The hypotheses that involve human action include competition, intergroup violence, or disease.

1) The first hypothesis that places AMH as the cause of Neandertal extinction is competition. Anatomically modern humans outcompeted Neandertals for resources and utilized the environment more efficiently. Early evidence suggested that AMH had a more varied diet that included plant foods as well as meat whereas Neandertals were exclusively carnivores. Furthermore, AMH used more complex tools that could kill game at a distance (La Pierre, 2008). In contrast, Neandertals had simpler weapons that put them up close with their prey. As a result, their mortality rate was suggested to be higher than that of AMH. However, recent evidence supports a more varied diet for Neandertals that included plant foods as well as meat (Brooks et al., 2011). Therefore, it is unlikely that completion was the primary cause of extinction, although it may have been a contributing factor.

2) Intergroup violence is the hypothesis that most directly links humans with Neandertal extinction. In essence, humans hunted down Neandertals and drove them to the brink of extinction. The surviving Neandertals were too scattered and too few in numbers to survive and reproduce. While the evidence for this is minimal, some fossil evidence exists. For example, there is a punctured rib in a Neandertal individual at Shanidar that resembles a projectile weapon used by AMH (Churchill et al., 2009). There are also other wounds that have been attributed to intergroup violence (McCall and Shields, 2008). Finally, in support of this hypothesis, intergroup violence fits the short time frame of co-existence that ended with Neandertal extinction.

3) Another hypothesis involves AMH bringing to Europe with them a virus that decimated the Neandertal population. After years of co-evolution in Africa, AMH had become resistant to the virus, but the Neandertal population was immunologically naïve (Greenwood and Wolff, 2010). Support for this hypothesis is largely based off historical evidence of foreigners bringing with them new diseases that killed off a large part of the indigenous population (i.e. Europeans bringing small pox to the New World and killing millions of Native Americans).

4) Other hypotheses have also been put forth that do not rely much on interaction between the two groups, such as humans being better adapted to the climate changes. However, whatever the case may be, it is clear that Neandertals remain an important part of human evolution. In conclusion, sequencing of the Neandertal genome has revealed that Asian and European modern human populations share 1-4 % of their genome with Neandertals. In contrast, present-day Africa populations do not have this genetic relation. Therefore, while the intergroup interbreeding was probably minimal, a part of the Neandertal genome exists today in the modern human population.

 The theory is that Homo’s all came from Africa and gave rise to the Neanderthals and Homo erectuses. Scientist have used the molecular clock as evidence that the out of Africa hypothesis is the best estimate/theory of how the humans of today came about. The molecular clock theory states that the rate at which proteins and DNA change relative to itself and is constant for whichever substance it is. For example, that rate at which HIV replicates and mutates is constant and the rate at which proteins are being replicated in humans and the DNA and RNA running the transcription then translation of the central dogma.

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