Grasses and the evolution of hominins

Introduction

Human evolution is a topic in the field of paleoanthropology, the study of the origin, diversification, and individual species in the tribe Hominini, whose members are called hominins. The subjects involved in the study of human evolution are very diverse. They include archaeology, which focuses on material remains left behind, like stone tools and beads; paleontology, which is the study of the physical remains of hominins (and other life forms) in the form of fossils; paleozoology and paleobotany, which are subdivisions of paleontology that focus on the animals and plants found at the same sites as hominin fossils; and paleoclimatology, which is the study of the ancient environments and landscapes in which hominins lived. Together, these fields of study allow scientists to gain a holistic picture of who our ancestors were and the types of environments in which they lived.

Modern humans, the species Homo sapiens, are the only living members of the tribe Hominini, a group that was once much more diverse millions of years ago. Members of the hominin group split from their closest living relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), around 6 to 8 million years ago somewhere in sub-Saharan Africa. Some of the traits that separate hominins from our ape relatives are the ability to walk on two legs, larger and more complex brains, and the ability to make and use tools.

One of the original theories for how these traits came to evolve was the Savanna Hypothesis, the hypothesis that early hominins developed the ability to walk on two legs to live in and see over grasses in large, open environments that were different than the closed forests that other apes live in. While support for this theory has waxed and waned over time—since it was likely not so clear-cut—the role of grasses and expanding grasslands are still central to our understanding of human evolution.

Evidence for ancient environments

Evidence from hominin fossils

One way to gain information about the environments that hominins lived in is by looking at the fossil hominins themselves. Their limb proportions, or how long their arms and legs are, can tell scientists if they were more likely climbing in trees or living on the ground. Longer arms generally means that they were spending more time in trees. The position of the head and spine as well as the structure of the hip and leg joints also tells scientists whether an animal walked on two legs or four. For human evolution, the lack of reliance on trees (shorter arms) and increased reliance on walking on two legs would indicate that they lived in more open environments like grasslands.

Evidence from stable isotopes

Carbon isotopes

Another form of evidence is isotopes, different forms of the same element with different numbers of neutrons in the nucleus of the atom. The element carbon, on which all life is based, has two stable forms: carbon-12 (¹²C) and carbon-13 (¹³C). Different plants process these isotopes of carbon in different ways depending on how those plants undergo photosynthesis. Plants that follow a C4 pathway include many grasses, whereas C3 plants include most trees, bushes, shrubs, and non-grass herbs (non-grass, non-woody plants). These plants have inherently different ratios of ¹²C to ¹³C because of their different photosynthetic pathways.

Oxygen isotopes

Another means of reconstructing past environments is by using the stable isotopes of oxygen (O), the element abundant in water. When the environment is cold, the lighter oxygen-16 (¹⁶O) isotope, which can evaporate easier, is trapped in glaciers on land, meaning that there is a higher abundance of the heavier oxygen-18 (¹⁸O) isotope in the water cycle. When temperatures are warm, ¹⁶O that is locked in melting glaciers is freed and rejoins the water cycle, thus lowering the relative abundance of ¹⁸O in the environment. Thus, oxygen isotopes can be used to reconstruct global temperatures.

The earliest hominins: Ardipithecus

One of the earliest creatures that is widely accepted as being part of the hominin lineage is Ardipithecus ramidus, which lived from 4.8 to 4.3 million years ago in what is now Ethiopia. This early hominin species is known best from a female skeleton named ‘Ardi’ discovered in 1994. Ardi exhibited some traits linking her to humans, such as hips that indicated bipedal (two-legged) walking, smaller canine teeth, and a skull that rested in an upright posture above the body rather than being held in front of the body like in four-legged animals. However, Ardi still had long arms and fingers like other apes, grasping big toes, and a brain size smaller than that of modern chimpanzees.

The way that hominins moved—or their locomotor adaptations—give scientists clues to the environments in which they lived. Ardi retained many ape-like features that made it well-suited to living in and around trees, like grasping big toes and long arms. However, it had already evolved other features, like its hips and head position, indicating that it spent a lot of time walking around on two legs. Altogether, the fossil evidence indicates that Ardipithecus may have been adapted to walking on the ground in more open environments than other apes, but that it still spent a lot of time in the trees.

Carbon isotope evidence for the environment that Ardi lived in come from animal fossils found at the same site as well as fossils of Ardipithecus fossils themselves. Overall, evidence from the site of Aramis, where Ardi was discovered, indicates a mix of grassland and woodland, or grassland with a variable, but important, level of woody cover. Similar isotopic data from another Ardipithecus site, Gona, is indicative of a woodland or bushland environment. In neither case is there isotopic evidence for closed-canopy forests, the type of environments in which living apes are most often found.

Regardless of environment, carbon isotopes from tooth enamel of Ardipithecus fossils themselves show a strong reliance on C3, or woodland/forest, food sources. This indicates that even if Ardi lived in more open environments, it likely spent most of its time in and around whatever trees may have been present.

The emergence of Lucy: Australopithecus

One of the most well-recognized fossils in human evolution is that of Lucy, belonging to the species Australopithecus afarensis, that lived from around 3.8 to 3 million years ago in eastern Africa. From the hips, knees, feet, and posture of Lucy, scientists are confident that this species was facultatively bipedal, meaning that it relied on bipedal walking as its main form of locomotion. In contrast, Aridpithecus could walk bipedally, but may not have done so consistently. Like Ardipithecus, Lucy’s species retains longer arms like other apes, leading scientists to debate whether Australopithecus afarensis still spent a lot of time in trees, or if their long arms were just an evolutionary remnant like tail bones in modern humans. Interestingly, Lucy herself is believed to have died from falling out of a tree.

A mixture of effective bipedal walking and the retention of longer arms likely served Lucy well for the types of environments she lived in. Based on carbon isotope data from Australopithecus afarensis sites from across eastern Africa, this species inhabited a wide range of habitats from woodland to open grassland and everything in between. Based on oxygen isotope data, it is clear the period in which Lucy lived was characterized by environmental instability, fluctuating cold periods (which would prompt grassland expansion) and warm periods (which would prompt forest expansion).

Paranthropus: nutcracker or hominin cow?

Around 2.5 to 2 million years ago, two new groups of hominins emerged, increasing the diversity of human forms present in Africa. One, the genus Homo, includes species that more closely resemble modern humans than other hominins in form or behavior. Our genus is the only one we know of to have left Africa by around 1.98 million years ago. The other, the genus Paranthropus, was characterized by wide faces, massive jaws, and large molars (cheek teeth). Three species of this genus are recognized from eastern and southern Africa.

Historically, the large teeth and jaws of Paranthropus fossils led scientists to hypothesize that they used their massive jaw muscles to crack nuts and eat other hard objects, causing the original fossil discoveries to be colloquially called “nutcracker man.” While the origins of these large jaws showed up in some earlier Australopithecus species, Paranthropus appears to have become much more specialized.

Analysis of carbon isotope data from Paranthropus fossils show that some have a strong C4 signal, indicative of a diet of grasses and sedges, while others have a more mixed feeding signal. Paranthropus boisei, from eastern Africa, is the most robust of the Paranthropus species, meaning that it has the largest teeth and strongest muscle attachments. Paranthropus boisei also has the strongest C4 signal of any hominin, indicating a diet of almost exclusively grasses and sedges.

Another form of evidence to assess diet is dental microwear, analyzing the small scratches and abrasions that food leaves on tooth enamel. Habitually eating hard objects would lead to cracks in the enamel surface. Habitually eating grasses and sedges would lead to more scrapes and microscopic changes to the enamel surface due to the silica (in the form of phytoliths, or silica bodies) and grit in these plants. The teeth of Paranthropus boisei much more closely match the microwear pattern for grass and sedge-eaters.

Altogether, the data support the idea that the large muscles of Paranthropus boisei were most likely used for consistent chewing of grasses and sedges rather than short-burst biting of hard objects. While these bony adaptations reached their height in Paranthropus boisei, there are traces of the same features in Paranthropus robustus in southern Africa, which is reconstructed as a more mixed feeder, and in earlier Australopithecus species. Therefore, grass and sedge consumption may be an integral part of ancient hominin diets, in contrast to the fruit-based diet of other apes.

Out of Africa: the origin and dispersal of Homo

Although ‘human’ characteristics like bipedal walking, larger brains, and the use of tools are present in earlier hominin species, these are most associated with the genus Homo. It was with the emergence of Homo erectus (sometimes called Homo ergaster) in Africa around 2 million years ago that brains start to get dramatically larger and that new types of tools for more specific purposes emerged. It may be no coincidence that Homo erectus is the earliest hominin species that is found outside of Africa.

Some of the earliest sites outside of Africa with evidence of hominin occupation include Dmanisi in Georgia (1.85 million years ago), Yuanmou, China (1.7 million years ago), and Sangiran on the island of Java in Indonesia (1.66 million years ago). While these sites were subject to the same climatic instability that characterized hominin evolution, all these sites are also reconstructed as being, in large part, grassland environments.

Whether hominins exited Africa across the Arabian sea or through the Sinai Peninsula is debated. Climatic reconstructions of the eastern Sahara show that wide grassland corridors and extensive lakes would have been present during wetter periods, allowing for migration out of Africa. The earliest archaeological evidence of boats or other overseas crossing is not until more recently, less than 100,000 years ago. While the lack of archaeological evidence does not automatically mean that crossing out of Africa via boat is impossible, an overland crossing would have been much more likely.