1. What’s Life? ASTR 1420 Lecture #2 Section 5.1

2. What’ Life? Have you ever wondered why all those aliens appeared in “Star Trek” or “Star Wars” movies look alike? To know what life may look like on other worlds, the best (only!) place to start to learn about life is here on Earth! So, let’s define “Life” based on living organisms on Earth!  “Life” in the context of astrobiology.

3. What’s Life? Different meanings of “Life” Different meanings of “Life” o a characteristic state or mode of living; "social life"; "city life"; "real life” o the course of human events and activities; "he could no longer cope with the complexities of life” o the condition of living or the state of being alive; "while there's life there's hope"; "life depends on many chemical and physical processes” o the period during which something is functional (as between birth and death); "the battery had a short life” or something closer to the astrobiological context… o living things collectively; "the oceans are teeming with life” o the organic phenomenon that distinguishes living organisms from nonliving ones; "there is no life on the moon"

4. Common characteristics of Life Biologists have identified at least six key features that appear to be shared by nearly all of living organisms on Earth. o Order o Reproduction o Growth and Development o Energy Utilization o Response o Evolutionary Adaptation

5. Characteristics of Life Biologists have identified at least six key features that appear to be shared by nearly all of living organisms on Earth. Order : all living organisms exhibit order in their internal structure. molecules in living cells are not scattered randomly but instead arranged in specific patterns to make cell structures. Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

6. Characteristics of Life Biologists have identified at least six key features that appear to be shared by most or all of living organisms on Earth. Order : all living organisms exhibit order in their internal structure. However, snow flakes, for example, show remarkable structures also. But, snow flakes are not alive! Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

7. Characteristics of Life Biologists have identified at least six key features that appear to be shared by most or all of living organisms on Earth. Order : all living organisms exhibit order in their internal structure. How about galaxies? Are they alive? Therefore, “order” is not sufficient condition for life. But, it is a necessary condition. Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

8. Characteristics of Life Reproduction: Organisms reproduce their own kind A single-celled organism (an amoeba) copying its genetic material (DNA) and dividing into two genetically identical cells. Mule, Tigon, Liger, Computer virus? Again, necessary condition, not sufficient! Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

9. Characteristics of Life Growth and Development : Living organisms grow and develop in patterns determined at least by heredity Growth and Development : Living organisms grow and develop in patterns determined at least by heredity Growing embryos of Costa Rican frog Heredity : passing on characteristics from one generation to next Wild fires, crystal? Again, necessary condition, not sufficient! Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

10. Characteristics of Life Energy Utilization: Living organisms use energy to fuel their activities Energy Utilization: Living organisms use energy to fuel their activities Tube worms near a deep sea volcanic vent. Car? Again, necessary condition, not sufficient! Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

11. Characteristics of Life Response: Living organisms actively responds to changes in its surroundings Response: Living organisms actively responds to changes in its surroundings A blacktail jackrabbit’s ears flush with blood, the blood automatically adjusted to maintain a constant internal temperature Mercury in a thermometer? Again, necessary condition, not sufficient! Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

12. Characteristics of Life Evolutionary Adaptation: Life evolves in a way to become adapted to its surrounding Evolutionary Adaptation: Life evolves in a way to become adapted to its surrounding white-tail ptarmigan white-tail ptarmigan Camo-moth Camo-moth counter example? Order Order Reproduction Reproduction Growth and Development Growth and Development Energy Utilization Energy Utilization Response Response Evolutionary Adaptation Evolutionary Adaptation

13. Evolutionary Adaptation 2 undeniable facts  an inescapable conclusion Heritable traits that enhance survival and reproduction will become progressively more common in succeeding generations! Charles Darwin (1858) Natural Selection Overproduction  struggle for survival Individual variation An imaginary population of beetles of mixed color evolves into dark beetles.

14. Evolution of Galapagos finches Evidence of Natural Selection Found them on remote, young, volcanic islands. They can only be found on Galapagos and they differ from the ancestor, and they differ on different islands.

15. Artificial selection Six vegetables that look & taste very different. But they were all bred by humans within just a few thousands years, from the same wild mustard.

16. Artificial Selection canines canines Artificial selection can cause profound changes in just over a few thousands years.  if artificial selection could cause such profound changes in just a few thousand years, natural selection could do far more over many billions of years!

17. Mechanism of Evolution Passing down hereditary traits from one generation to next is done through DNAs. Passing down hereditary traits from one generation to next is done through DNAs. Any change in DNA sequence (at the level of 1 in billion) can cause a mutation and introduce a new trait in the ecosystem. Then, this new trait is exposed to the natural selection process. Any change in DNA sequence (at the level of 1 in billion) can cause a mutation and introduce a new trait in the ecosystem. Then, this new trait is exposed to the natural selection process. Example: possibly the first life on the Earth (self-reproducing RNA)  RNA that could reproduce faster than its competitors survived  then, to maximize the effect of RNA, a type of RNA variation with a cellular structured can outperform its competitors.

18. Good definition of Life? Life is something that can reproduce and evolve through natural selection This looks like a good definition of life at least for terrestrial life.  Do living things on other worlds evolve also?  If so, how can we see such evolutions so far away? Can we be sure that life on other worlds would resemble life on Earth physically or chemically. In the Universe, we should expect quite exotic, extraordinary, unimaginable types of life. We will look into this possibility during the lecture #9 (What types of life can we find?).

19. In summary… Important Concepts Characteristics of Life o Examples and counter examples for each characteristics Definition of Life Evolutionary Adaptation Natural selection Important Terms Heredity Necessary condition Sufficient condition Artificial selection Chapter/sections covered in this lecture : section 5.1 & 5.2

20. Defining Life What is Life? Seems to be a simple question, but it is not! Life : (from Oxford dictionary) the condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death 1. growth : Crystal, fire, etc. can grow... 2. reproduction: computer viruses can reproduce… 3. functional activity: nearly all man-made appliances are active… 4. change: stars, mountains, rivers are changing continuously…  A single term can’t define life sufficiently Defining “Life” is not easy!