Time

Think of all of the Investigations in which we plotted time on a graph? For example, chemical reaction product formation over time, distance plotted against time giving us velocity, or gas temperature over time. In fact, time is quite probably one of the most common variables plotted on scientific graphs. Perhaps nowhere does the record of time, however, figure with so great importance as in tracking the origin, duration, and extinction of living species over geologic time.

The graphic below represents the characteristics and size of the Univers over the most expansive time span conceivable. At the very far right, the time is the present – the time we live in. On the far left, approximately 13.7 billion years ago, marks the actual beginning of time! The current theory places the Big Bang event at this far distant beginning of time.

Not only did time begin at the Big Bang, but all matter and energy in the entire Universe burst into existence from nothing at exactly that point as well. Everything you have learned about matter and energy is consistent with this concept. We know that neither matter nor energy can be created or destroyed (the Law of Conservation of Matter and the Law of Conservation of Energy, respectively). Therefore, all matter and energy that exists throughout the entire Universe have always been exactly the same since the Big Bang. What happened before the Big Bang or why the Big Bang occurred, is currently anybody’s guess. We can not go further back into time than 13.7 billion years, because time simply didn’t exist before. This is perhaps the most difficult concept in all of science to grasp. Before the Big Bang, there was nothing. That is, the Big Bang occurred on a day that had no yesterday! 

The Big Bang graphic above indicates that the Earth formed about 4.5 billion years ago and that life first appeared on Earth about 3.5 billion years ago. Thus, when we speak of topics like adaptation, natural selection, and evolution – the focus of this CELL – we must begin by thinking of time in terms that you have probably never thought of before. Life on Earth has had 3.5 billion years to arrive at its current state. Continents have drifted around the globe like leaves on a pond since life began on Earth. Ice ages have repeatedly come and gone and our planet has incurred countless impacts from extraterrestrial bodies like meteors, comets, and asteroids. Throughout all of this, life has adapted and changed in response to major and minor environmental changes. And life continues to adapt and change today, and will into the distant future.

This expanded conception of time is of fundamental importance in the study of biological adaptation and evolution because the mechanisms by which life on Earth has changed with time can occur very, very, very slowly and yet still have dramatic results. On the other hand, certain cosmic events can cause profound changes in a flash of a moment, like when a large asteroid collided with our planet only 65 million years ago – an event that marked the extinction of the dinosaurs.

The statement above, “only 65 million years ago”, may cause you to ask, “How can we use the term only when referring to 65 million years”? A brief overview of life on Earth over time is shown in a unique format in the spiraling graphic below. As you can see, 65 million years is little more than the first quarter of a turn along the spiral back into time from the present. Sixty-five million years, in terms of the beginning of life on Earth, let alone since the formation of the Earth or the Big Bang, is a relatively recent event. Notice that detail can only be seen as far back as about 550 million years ago, in the Cambrian Period. If you click on the illustration you can examine a much larger image in a new window. Look at how the spiral disappears into the past toward the lower left of the illustration.

Also, notice how recently humans have arrived as a form of life on Earth. We have only been on the scene for perhaps up to 2.5 million years, during the Quaternary Period. That is the most recent period of time shown in this graphic. It is basically the present. Let’s keep this sense of the vastness of time in mind as we begin to think about life and adaptation of life in this CELL.

What is a Species?

A species is a very closely related group of living organisms. In biological terms, it is the most basic grouping of organisms since one practical definition of a species is a group of organisms so similar that they can mate with each other and produce viable offspring. All humans are of the same species, Homo sapiens. Even though we may be similar in many respects to other species of animals, particularly primates, we are our own species.

Each species consists of individual organisms that appear more similar to one another than to the individuals of another, different species. However, each individual in a species is slightly different. In the case of humans, for example, some people have blue eyes, others have brown or green. Some people are taller than others and some people have slightly different immunity to certain diseases. Some of these differences are obvious while some are not. The fact that each species consists of individual organisms that are slightly different from one another is a consequence of the normal genetic variation within a species. Again, our own species Homo sapiens includes individuals who are very different in appearance from other human beings but appear more similar to each other than to individuals of any other primate species. A similar physical appearance or phenotype is due to the contribution of many different traits, (e.g. eye color, height, skin pigmentation, endurance, and so on) all of which are similar but not identical for a species.

The genotype of a species is the combination of all the different genes that determine a species’ phenotype. The genotype of a species determines the phenotype of that species. In the simplest case, each trait is determined by one of two different alleles of a single gene, one dominant allele, and one recessive allele. In reality, most traits are so complex that each trait is determined by a combination of alleles of many different genes. You will learn more about genes, traits, genotypes, and phenotypes in all three Investigations of this CELL. Since the phenotypes within a species are slightly different, it is reasonable to assume that the genotypes within a species are also slightly different. These slight differences in both phenotype and genotype are a result of genetic variation.

The environment of a species includes factors such as weather, temperature, food supply, water supply, and the presence of other organisms. All of these factors have a direct effect on the well-being of a species. Changes in one factor of a species’ environment over either a short or long time interval affect whether a species will effectively survive and reproduce. Adaptation of the species occurs via an increase in the proportion of the individuals that possess a trait that permits survival. Adaptation to environmental pressure is necessary if the species as a whole is to survive. If the species cannot survive an environmental change, for example, all individuals will perish and the species will become extinct.

Traits and Natural Selection

As discussed, each individual within a species is slightly different because all of the traits of each individual are slightly different. When an environmental factor changes and environmental pressure is exerted, one of these slightly different traits may offer an important survival advantage to the individuals that possess the trait. As these individuals reproduce, the trait is passed to the individuals’ offspring, providing a survival advantage to new individuals as well. Those individuals without the advantageous trait may not survive a certain environmental change – a new environmental pressure – but those with the trait survive and reproduce until all individuals of the species possess the particular gene and exhibit the advantageous trait. What was once genetic variation is now a gene common to all individuals of the species. The environmental pressure on the individuals of a species results in the adaptation of the species as a whole, a process that is termed Natural Selection.

Fossils and Paleontology

As individuals of a species die, their fleshy remains decompose but the large bones of their skeletons remain. Through the replacement of the bone with long-lasting minerals, the bony remains of a species may fossilize and can be found millennia later in the form of fossils. Not all of the bones of a skeleton are typically found due to scavenging of the remains and erosion which separate the smaller from the larger bones. Usually only large, dense bones such as the femur, pelvis, and skull survive the process of fossilization. Occasionally the fossils of whole skeletons can be recovered if an organism is covered by sediment shortly after its death.

As time slowly passes over thousands of years, layers of soil and debris cover the fossilized remains of organisms from different time periods forming a vertical record of the extinction of many species. More recent remains are continuously deposited on top of the remains laid down earlier. By excavating vertically from top to bottom, a record of the extinction of many species can be constructed from recently extinct species to those species that became extinct long ago. The process of establishing the fossil record and constructing a timeline of extinction is studied by paleontologists.

Up to this point, we have discussed how organisms adapt to environmental pressure and change over time. We also discussed the timeframe involved in the slow process of adaptation and natural selection. In these Investigations, we will explore these topics in greater detail and spend time in the lab creating and testing models that will illustrate these important concepts. Importantly, we will develop a greater understanding of how the traits that are responsible for the genotype and phenotype of organisms are ultimately dictated by genes on chromosomes that are passed from parents to their offspring.

It is important for you to realize that very similar mechanisms are involved not only in animals but in plants as well. The plants that we have today did not always exist in the form that we see them. For example, flowering plants, which include so many present-day shrubs, bushes, and trees, did not appear until about 130 million years ago. By examining the spiraling timeline above, you will find that, while the first dinosaurs to appear lived in the absence of flowering plants, dinosaurs that evolved later, like Triceratops and Tyrannosaurus, lived their entire lives around flowering plants. Plants and animals have evolved on the Earth together, often exerting environmental pressure on each other.

Hominid Fossil Record: Human Adaptations

Hominids are ancient, pre-human primates. Changes that have occurred in humans and our pre-human ancestors have occurred over about 4 million years of geologic time. In the brief overview summarized in the graphic below, you can hopefully see from the skull fossils, that arriving at our current form as modern humans (Homo sapiens) has been a long, slow process. To see details better, simply click on the image and it will open in a new window.

One of the most important observations one can make studying the skull chart above is that modern humans are not decedents of modern apes, represented by the chimpanzee skull in this timeline. This is a common misconception. Rather, modern chimpanzees and modern humans branched from a common ancestor (called hominids) over 5 million years ago. Both of us have evolved separately since then.

Unlike many other animals, Homo sapiens adaptations, natural selection, and evolution have focused not only on physical traits like our opposable thumbs and upright, bipedal walk but also on cognitive advances in the human brain. The very size of our brain separates us from the species that preceded us. Below is an interesting graphic comparing the brain size of the various species that were listed in the evolutionary tree in the previous illustration.

Notice that the modern chimpanzee brain size is similar to prehuman animals that we evolved from over 4 million years ago! Also, notice that the species Homo neanderthalensis, a species that briefly overlapped with modern humans, had a brain size comparable to or even slightly larger than modern humans (Homo sapiens). However, it is difficult to know precisely which parts of the brain were developed in extinct species such as the neanderthals since brain tissue is rarely fossilized for paleontologists to study. Modern humans have a very highly developed and large frontal lobe of our brain. This is the part of the brain that is located directly behind our foreheads and is responsible for many of the higher-level thinking processes that are exquisitely developed in our species.

Undoubtedly, physical traits like opposable thumbs lead to tool-making. However, tool-making abilities existed in prehuman species that later became extinct. Homo sapiens, because of their highly developed cognitive skills took tool-making from simple hand axes made of chipped stones to spears, bow and arrows, plows, trains, airplanes, televisions, and smartphones. Thus, it is our highly advanced brains that facilitated our adaptation to environmental pressures over the past hundreds of thousands of years and permitted us to survive, pass traits on to our offspring, and spread across the entire surface of the Earth.

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

Investigation 1:

• Why do individuals of a species have different traits?  The genotype of an individual determines the phenotype of the individual. Genetic variation ensures that there are many possible genotypes and phenotypes that an organism can possess. This genetic variation is a result of mutations, recombination of genes, and the random mating of individuals.

• What is the relationship between the survival of the individuals of a species and genetic variations? Genetic variation is the reason why there are many unique individuals of a species in a population. Genetic variation allows a population of species to survive different environmental pressures.

Investigation 2:

• How does genetic variation affect the type of traits or the adaptations in the individuals of a species? Genetic variation is responsible for all the different traits that an individual possesses.

• What is the relationship between genetic variation and natural selection? If members of a species did not have different traits no natural selection could exist because all the members of that species would be equally adapted to each environmental pressure. However, there is genetic variation in populations and some individuals are more adapted to their surroundings than others. The ones that survive are more likely to pass on their genes.

• How can an environmental change affect a population? Environmental pressures can affect the population because some members are more adapted to the new environment than others.

Investigation 3:

• How can an environmental change affect a population? When a new environmental pressure is introduced, some organisms in a population will not survive. The organisms with the genes that are most adapted to the environment will have the highest chance of surviving and passing their genes on to the next generation.

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Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.

Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.