id="ulink_886835bf-3f62-5d6c-ac99-f8abd21330ed">
1.1 A Brief History of Man and Ice
1.1.1 Ice on Earth
Planet Earth is in a constant, albeit slowly, changing state. In the 4.5 billion years since its formation, the climate of Earth has gone through many fluctuations in which there have been periods of more or less ice accumulation. Upon formation, during the Hadean Eon, young planet Earth was completely molten and had frequent collisions with other bodies in the primordial solar system [1]. Many of these colliding bodies were “planetary embryos” which were carriers of water, thought to have originated from the outer asteroid belt [2]. The accumulation of water around the molten blob that was Earth allowed for cooling to occur and a solid crust to form approximately 4.4 billion years ago. One of the collisions with another celestial body caused the formation of the Moon, and in the same process created Earth’s early CO2- and water vapour-rich atmosphere. The high pressure of this young atmosphere allowed for the first time liquid water oceans to exist on Earth, despite the estimated surface temperature of 230°C [3].
Single-cellular life emerged in Earth’s oceans approximately 4 billion years ago, marking the start of the Archean Eon [4]. Oceanic cyanobacteria, the first single-celled organisms capable of photosynthesis, appeared approximately 3.5 billion years ago and started producing oxygen as a waste product. The oxygen produced through photosynthesis was readily captured chemically by dissolved iron until approximately 2.4 billion years ago when these oxygen stores became filled, at which point atmospheric oxygen appeared for the first time [5]. This Eon, known as the Proterozoic, also saw volcanic activity diminish which lessened the levels of atmospheric CO2. The lowered CO2 and increased O2 concentrations resulted in a weakened greenhouse effect, leading to the Huronian glaciation [6]. This marks the first time that water existed as a solid on Earth. The approximately 300 million year long event was severe, with the entire planet being frozen over in what has been termed “snowball Earth”. The Huronian Ice Age was one of the main contributing factors to planet Earth’s first mass extinction [6, 7].
Earth’s temperature rebounded approximately 2.1 billion years ago, allowing for the first Eukaryotic cells to form. This was followed by the emergence of multicellular organisms approximately 1.6 billion years ago [8]. As multicellular organisms began to grow larger, and more complex, their death (and subsequent sinking to the seabed) is hypothesized to have served as a sequestration of CO2 [9, 10]. The sequestration of CO2 caused the onset of the Cryogenian period 720 million years ago, a period which included the longest known ice ages: the Sturtian and Marinoan glaciations [11]. The post-Cryogenian Earth has seen the evolution of Molluscs and Anthropods, in what has become known as the Cambrian Explosion due to the gigantic number and diversity of the forms of life introduced [12, 13]. The Andean-Saharan glaciation marked the next major extinction event on Earth, approximately 450 million years ago and lasted about 30 million years [14]. The end of the Andean-Saharan ice age ushered in the evolution of the first Tetrapoda, Earth’s first land animals as well as an explosion in the number of land plants [15, 16]. The huge number of land plants greatly increased the level of atmospheric oxygen, and decreased the level of atmospheric CO2, leading to another period of a weakened greenhouse effect. Thus started the Late Paleozoic Ice Age approximately 360 million years ago which would last approximately 100 million years [17].
The rebound in global temperatures following the end of the Late Paleozoic Ice Age saw the introduction of large land animals, most notably the Dinosaurs in the late Triassic period approximately 240 million years ago [18]. Following the extinction of the Dinosaurs about 66 million years ago, mammals, birds and flowers evolved, again affecting the level of atmospheric CO2. The decreased greenhouse effect of the present time started approximately 2.6 million years ago, leading to what is called the Quaternary (or Current) Ice Age [19]. Periodic changes in the Earth’s orbit around the sun during this glaciation has resulted in characteristic interglacial (or warm) events [20]. The latest such retreat of the ice sheets 12000 years ago has been called the Holocene Epoch, and has allowed humans to inhabit northern latitudes [21].
1.1.2 Man is Carved of Ice
It is evident from Figure 1.1 that life and ice on Earth have had a symbiotic relationship. The emergence of new life on the planet has contributed to changes in the atmosphere, lessening the greenhouse effect, and leading to periods of glaciation (including the very first appearance of ice on the planet). The evolutionary selection pressures of drops in global temperature led to mass extinction events where the only organisms that survived were those fit to live in the cold. The cold climate selection pressures, among many others, led to the evolutionary emergence of more complex Eukaryotic cells, and eventually multicellular organisms which in death sequestered CO2, leading to more glaciation. This cycle continued with the introduction of land plants and animals, Dinosaurs, mammals, birds and flowers, until the current geologic period of the Earth - the Quaternary -was reached.
Figure 1.1 A geologic timeline of the Earth shows the symbiotic relationship between life and ice on the planet. The inset of the past 9 million years shows the specific relationship between glaciation of Earth (through oxygen-18 isotope measurements) and human evolution. δ18O data is taken from [22]. bya and mya denote billons of years ago and millions of years ago, respectively.
Climate fluctuations continue to affect the evolution of living things; the evolution of living things continues to affect the climate. In fact, the climatic conditions in the Quaternary Ice Age, including those which present the conditions for ice formation, have shaped the selection pressures experienced by early humans, and thus human evolution [23]. Although the first Primates evolved approximately 60 million years ago, the earliest Apes appeared on Earth approximately 20 million years ago [24]. As Earth entered the Neogenic and Quaternary periods, the characteristic dramatic climatic fluctuations of these geological time periods ushered in the evolution of the first Homininae, the tribe including Gorillas, Chimpanzees and Humans [25]. Earth’s climate throughout the Pliocene (beginning approximately 5.3 million years ago), Pleistocene (beginning 2.5 million years ago), and Holocene (beginning 12000 years ago) epochs has trended towards not only a colder more glaciated world, but also increased amplitude and frequency between cool and warm periods, as determined through oxygen isotope analysis of ice core samples [22]. Many of the major developments in hominin evolution have been coincident with the intensification of Northern Hemisphere glaciation, as shown in Figure 1.1 [26].
Adaptations to the increasingly unpredictable and unstable environments of the late Neogene and Quaternary periods might in fact be part of what it means to become a human in the Darwinian sense [26, 27]. The morphological and behavioural traits of terrestriality, bipedalism, dietary flexibility, tool use, omnivory, large brains, long-distance migrations, and the birth of dependant children, thought to characterize Homo sapiens have all been linked to fluctuations in Earth’s climate [23]. The environmental selection pressures led to evolutionary bottlenecks that, evidently, only our ancestors were able to pass through. Tool use and increased brain size specifically have been linked to periods of glaciation. The earliest evidence for the use of stone tools has been found in the Ethiopian region of Gona, where artifacts have been dated to 2.6 million years ago [28]. The first known use of tools coincides with the onset of Northern Hemisphere Glaciation 2.7-2.5 million years ago [29, 30]. As resources became scarce with falling temperatures, only those early human ancestors capable of constructing and utilizing tools for increased efficiency survived. Dramatic increase in the size and complexity of the brains of human ancestors, a process known as encephalization, also began around 2.6 million years ago. The
