What is life?

“A self-sustaining chemical system capable of Darwinian evolution.” – NASA definition of life

Learning Objectives

  1. Identify the common features of life on earth
  2. Distinguish living organisms from non-living entities
  3. Explain evolution as en emergent property of life
  4. Name the 3 domains of life

Life on Earth

We are intimately familiar with life around us, because we’ve seen and interacted with them all our lives. But some forms of life went unrecognized for most of human history because they were impossible to see until microscopes were invented. Other living organisms may be difficult to recognize, even when seen, as in this picture below.

Lithops is a genus of succulent plants (like cacti) native to South African deserts. Lithops are often called ‘living stones’ because they look like surrounding rocks, excellent camouflage if you are a delicious desert plant. Image from http://harrywoolner.wordpress.com/plants/lithops/

So how do we distinguish life or non-life? What are the attributes that all living organisms on Earth share? Which of these attributes are absent in non-living organisms, and exclusive to living organisms?

In addition to life on Earth, what about the possibility of life on other planets, with environments utterly unlike our planet? Suppose that we launch an exploratory mission to Mars or Europa (link to article on NASA mission to Europa, a moon of Jupiter with evidence for briny seas and hydrothermal vents beneath a layer of ice), and can send a laboratory module equipped with any type of analytical instrumentation you can think of. How would you search for evidence of life on Mars or Europa?

If we look at the fundamental properties of life, what might be some emergent properties of life?

These questions (the properties of life) form the major topics of this course: Introduction to Biological Principles.

There are five generally agreed upon criteria for life:

  1. Need for energy
  2. Organization in membrane-bound cells
  3. Genetic information
  4. Ability to replicate
  5. Change (in a population) over time

Evolution as an emergent property of life

A key part of any definition of life is that living organisms reproduce. Let’s now add a couple of observations:

  • The process of reproduction, while mostly accurate, is imperfect. When cells divide, they have to replicate their DNA. Although DNA replication is highly accurate, it still makes about 1 mistake in 10 million nucleotides. Over generations, the population will contain lots of heritable variation.
  • The population of a given type of organism will tend to grow exponentially, but will reach a limit, where the individuals have to compete with each other for the limiting resource (food, space, mates, sunlight, etc.)

Suppose some heritable variations (speed, strength, sharper claws, bigger teeth) make some individuals more competitive for the limiting resource – what will happen?

The individuals with superior variants will acquire more resources, and have more progeny. If the superior variants are heritable, then their progeny will have the same superior variants. Over generations, then, a larger and larger proportion of the population will consist of individuals with the superior heritable variants. This is biological evolution.

Definition: Biological evolution is change in the heritable characteristics of a population over succeeding generations. In more technical terms, evolution is defined as change in the gene pool of a population, measurable as changes in genetic variant (allele) frequencies in a population.

Suppose there is heritable variation in a population, and the heritable variation makes a difference in the survival and reproduction of individual organisms. If these conditions exist, and they do for all natural populations of living organisms, evolution must occur. Life evolves!

Charles Darwin called this process natural selection. He and Alfred Wallace were the first to propose that evolution by natural selection could explain the origin of all the multitudes of species on Earth and how they appear so well-adapted in form and function to their particular environments. Moreover, Darwin proposed that all of life on Earth descended from a common ancestor, via slow, incremental accumulation of heritable (genetic) changes.

Because the definition of evolution is change in the heritable characteristics of a population over generations, evolution can occur by means other than natural selection. Evolution can also occur via random processes, especially in small populations, where the frequency of some heritable traits may rise or fall just by chance. We will discuss these mechanisms of evolution in a future class session.

Three domains of life

DNA sequence comparisons and structural and biochemical comparisons consistently categorize all living organisms into 3 primary domains: Bacteria, Archaea, and Eukaryota. Both Bacteria and Archaea are prokaryotes, single-celled microorganisms with no nuclei, and Eukaryota includes us and all other animals, plants, fungi, and single-celled protists – all organisms whose cells have nuclei to enclose their DNA apart from the rest of the cell. The fossil record indicates that the first living organisms were prokaryotes (Bacteria and Archaea), and eukaryotes arose a billion years later.

Tree of life showing the three domains of life. Image credit: UC Museum of Paleontology (http://evolution.berkeley.edu)

People (yes, that includes you, dear reader) are made up of cells from all 3 domains of life. Our bodies have about 37 trillion (3.7 x 10^13) human cells (hosts of the DNA inherited from our parents), and about 100 trillion (1 x 10^14) bacteria, mostly in the gut (American Society for Microbiology, Human Microbiome FAQ). We also have Archaea, primarily methanogens (responsible for flatulence!), though they appear to comprise less than 1% of our intestinal microflora (Lurie-Weinberger MN, Gophna U, 2015).

How do we organize all these organisms?

See the Crash Course take on taxonomy, the classification system for all life on Earth: