Grades 9-12 or Adult Sophisticated Learner
How do we know that something is alive? The answer seems obvious and intuitive, but when you ask that question in the context of searching for life on other planets, it becomes more difficult to pin down. In the unfamiliar environments of other planets, we need to find a way to distinguish between a phenomenon that isn’t alive as compared to one that is. One way to address this is by considering the characteristics of living things, which reveals how much all living things have in common. All life on Earth adheres to a common biochemistry. In all living things, cells are the main unit of organization, cellular membranes are made up of molecules called phospholipids, genetic information is made up of molecules called nucleic acids, and functions within and between cells are mostly carried out by molecules called proteins. This means that a fly has the same basic biochemistry as an elephant!
Beyond this shared biochemistry, all life has certain general traits in common, too. Here are some of the key traits of life as we know it:
1) All life is highly ordered and structured. Not only do all living things that we know of have cells and cellular structures, but many living things also have larger-scale structure such as bilateral symmetry (in humans) or radial symmetry (in starfish).
2) All life reproduces itself, either sexually (as animals do) or asexually (such as budding in yeast or one cell splitting into two identical daughter cells via binary fission as bacteria do).
3) All life grows and develops to reach maturity, such as from a caterpillar to a butterfly.
4) All life takes in and utilizes energy to carry out the functions of its cells, which results in growth and development. Mechanisms for energy intake are vastly different across all species, and can range from eating food like humans do, to converting sunlight into sugars like plants do, to the harnessing of the energy produced when rocks radioactively decay like some bacteria do.
5) All living things exhibit homeostasis, which is the ability to maintain a steady internal environment regardless of their external environment. For example, most humans maintain a body temperature of 98.6 degrees Fahrenheit regardless of whether they are out playing in the snow or hiking in the hot desert. Homeostasis is achieved because of strict biochemical regulations in cells and organs.
6) All living things respond to their environment by sensing external stimuli and changing their biochemistry and/or behavior. For example, when cuttlefish sense danger, they can instantaneously change their colors to match whatever background they are against to avoid being seen by a predator.
7) Finally, all living things adapt to external pressures, and evolve because of them. Adapting is much like responding to a stimulus in the environment, but takes it to the next level. In evolutionary adaptation, one cuttlefish will have the ability to change colors more quickly and effectively than another (because of its genetic makeup), and it will inherently be more likely to survive than another one that doesn’t do it as well or as quickly. The first one is more likely to pass on its genes to its offspring, and that offspring will pass it on to their offspring, and so on. Over time, the population of cuttlefish descended from that one who changed colors more quickly and effectively is more highly adapted to its environment. They have undergone the process of natural selection and are more likely to survive. Their genes were “selected for” by the external pressures of the environment.
Something that is alive will exhibit all of these traits, while phenomena that we do not consider to be alive can exhibit some, but not all of them. For example, a fire exhibits some of these traits – it consumes energy (wood and oxygen) and gives off by-products such as CO~2~ and heat, it grows in size as it consumes more and more fuel, and it may appear to reproduce as it spreads. But because it doesn’t exhibit all of these traits, we don’t consider fire to be alive. Defining life from the viewpoint of examining its characteristics reveals how much life on Earth has in common, and helps distinguish between living and non-living things. If there is other life out there in the cosmos and it’s like the life that we know, then we would expect it to also show these traits of living things.
Disciplinary Core Ideas
LS1.A: Structure and Function: All cells contain genetic information in the form of DNA molecules. Genes are regions in the DNA that contain the instructions that code for the formation of proteins. (HS-LS1-1)
LS1.B: Growth and Development of Organisms: In multicellular organisms individual cells grow and then divide via a process called mitosis, thereby allowing the organism to grow. The organism begins as a single cell (fertilized egg) that divides successively to produce many cells, with each parent cell passing identical genetic material (two variants of each chromosome pair) to both daughter cells. Cellular division and differentiation produce and maintain a complex organism, composed of systems of tissues and organs that work together to meet the needs of the whole organism. (HS-LS1-4)
LS1.C: Organization for Matter and Energy Flow in Organisms: The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen. (HS-LS1-5) The sugar molecules thus formed contain carbon, hydrogen, and oxygen: their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA), used for example to form new cells. (HS-LS1-6) As matter and energy flow through different organizational levels of living systems, chemical elements are recombined in different ways to form different products. (HS-LS1-6, HS-LS1-7)
LS2.B: Cycles of Matter and Energy Transfer in Ecosystems: Photosynthesis and cellular respiration (including anaerobic processes) provide most of the energy for life processes. (HS-LS2-3)
LS2.D: Social Interactions and Group Behavior: Group behavior has evolved because membership can increase the chances of survival for individuals and their genetic relatives. (HS-LS2-8)
LS3.A: Inheritance of Traits: Each chromosome consists of a single very long DNA molecule, and each gene on the chromosome is a particular segment of that DNA. The instructions for forming species’ characteristics are carried in DNA. All cells in an organism have the same genetic content, but the genes used (expressed) by the cell may be regulated in different ways. Not all DNA codes for a protein; some segments of DNA are involved in regulatory or structural functions, and some have no as-yet known function. (HS-LS3-1)
LS3.B: Variation of Traits: In sexual reproduction, chromosomes can sometimes swap sections during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation. Although DNA replication is tightly regulated and remarkably accurate, errors do occur and result in mutations, which are also a source of genetic variation. Environmental factors can also cause mutations in genes, and viable mutations are inherited. (HS-LS3-2) Environmental factors also affect expression of traits, and hence affect the probability of occurrences of traits in a population. Thus the variation and distribution of traits observed depends on both genetic and environmental factors. (HS-LS3-2, HS-LS3-3)
LS4.A: Evidence of Common Ancestry and Diversity: Genetic information provides evidence of evolution. DNA sequences vary among species, but there are many overlaps; in fact, the ongoing branching that produces multiple lines of descent can be inferred by comparing the DNA sequences of different organisms. Such information is also derivable from the similarities and differences in amino acid sequences and from anatomical and embryological evidence. (HS-LS4-1)
LS4.B: Natural Selection: Natural selection occurs only if there is both (1) variation in the genetic information between organisms in a population and (2) variation in the expression of that genetic information — that is, trait variation — that leads to differences in performance among individuals. (HS-LS4-2, HS-LS4-3) The traits that positively affect survival are more likely to be reproduced, and thus are more common in the population. (HS-LS4-3)
LS4.C: Adaptation: Evolution is a consequence of the interaction of four factors: (1) the potential for a species to increase in number, (2) the genetic variation of individuals in a species due to mutation and sexual reproduction, (3) competition for an environment’s limited supply of the resources that individuals need in order to survive and reproduce, and (4) the ensuing proliferation of those organisms that are better able to survive and reproduce in that environment. (HS-LS4-2) Natural selection leads to adaptation, that is, to a population dominated by organisms that are anatomically, behaviorally, and physiologically well suited to survive and reproduce in a specific environment. That is, the differential survival and reproduction of organisms in a population that have an advantageous heritable trait leads to an increase in the proportion of individuals in future generations that have the trait and to a decrease in the proportion of individuals that do not. (HS-LS4-3, HS-LS4-4) Adaptation also means that the distribution of traits in a population can change when conditions change. (HS-LS4-3) Changes in the physical environment, whether naturally occurring or human induced, have thus contributed to the expansion of some species, the emergence of new distinct species as populations diverge under different conditions, and the decline – and sometimes the extinction – of some species. (HS-LS4-5) Species become extinct because they can no longer survive and reproduce in their altered environment. If members cannot adjust to change that is too fast or drastic, the opportunity for the species’ evolution is lost. (HS-LS4-5)
LS4.D: Biodiversity and Humans: Biodiversity is increased by the formation of new species (speciation) and decreased by the loss of species (extinction). (HS-LS2-7) Humans depend on the living world for the resources and other benefits provided by biodiversity. But human activity is also having adverse impacts on biodiversity through overpopulation, overexploitation, habitat destruction, pollution, introduction of invasive species, and climate change. Thus sustaining biodiversity so that ecosystem functioning and productivity are maintained is essential to supporting and enhancing life on Earth. Sustaining biodiversity also aids humanity by preserving landscapes of recreational or inspirational value. (secondary to HS-LS2-7, HS-LS4-6)
Stability and Change: Much of science deals with constructing explanations of how things change and how they remain stable. (HS-LS2-6, HS-LS2-7)
Big Ideas: All living things have certain traits in common: Cellular organization, the ability to reproduce, growth & development, energy use, homeostasis, response to their environment, and the ability to adapt. Living things will exhibit all of these traits. Nonliving things may exhibit some, but not all, of these traits.
Boundaries: Grade level appropriate examples of maintenance of homeostasis include heart rate response to exercise, stomate response to moisture and temperature, and root development in response to water levels. (HS-LS1-3)
4-12 Finding Life beyond Earth, Activity 2: What is Life? Page 16. In this activity, students observe a number of objects, make a list of life’s characteristics, and develop a working definition of life. https://d43fweuh3sg51.cloudfront.net/media/assets/wgbh/nvfl/nvfl_doc_collection/nvfl_doc_collection.pdf
6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include DNA and the Genome (page 15) and An Organism Based upon Arsenic not Phosphorus (page 17). NASA. https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf
6-12 Astrobiobound! Students create a space mission which requires them to balance the return of their science data with engineering limitations such as power, mass and budget. Astrobiobound engage students by giving them the opportunity to identify a significant target of interest in astrobiology and allowing them to plan their own NASA mission within our Solar System. This simulation follows the same considerations and challenges facing NASA scientists and engineers as they search for life in our Solar System and as they try to answer the compelling question, “Are we Alone?” NASA/Arizona State University. https://marsed.asu.edu/lesson-plans/astrobiobound
6-12 Astrobiology Education Poster and Activities: What is Life? Where is it? (Activity 1-2) and How do we find it? (Activity 3). With gorgeous graphics, supporting background reading, and three inquiry and standards-based, field-tested activities, this poster is a great addition to any middle or high school classroom. It explores the connection between extreme environments on Earth, and potentially habitable environments elsewhere in the Solar System. NASA.
8-10 SpaceMath Problem 392: Exploring the DNA of an organism based upon arsenic. Students estimate the increase in the mass of the DNA from an arsenic-loving bacterium in which phosphorus atoms have been replaced with arsenic. [Topics: integer math; percentages] https://spacemath.gsfc.nasa.gov/astrob/7Page57.pdf
9-10 Voyages through Time: Origin of Life. Through the Origin of Life module, students address questions such as: What is life? What is the evidence for early evolution of life on Earth? How did life begin? Sample lesson on the website and the curriculum is available for purchase. SETI. http://www.voyagesthroughtime.org/origin/index.html
10-12 The Rules of Life. This podcast covers how we predict the phenotype, the structure, function and behavior of an organism, based on what we know about its genes and environment. If we can identify some of the basic rules of life across scales of time, space and complexity, we may be able to predict how cells, brains, bodies and biomes respond to changing environments. NSF. https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=242752&WT.mc_id=USNSF_1