what is life? -- 6/5/24

05 Jun 2024

what is life? -- 6/5/24

0 COMMENTS

Today's excerpt-- from Alien Earths by Dr. Lisa Kaltenegger. The challenge of defining life:

“When you look for life, what signs can you search for? What characteristics make something alive? In other words, what is life? It is surprisingly hard to define. For example, you can say that life moves. But so does fire. You can say that life evolves. But a computer virus can evolve. Another criterion is that life reproduces—but does that mean that mules (which are sterile) are not alive? You can see how hard it is to define what we’re searching for.

“We don’t have a good definition for life yet, not one that scientists can agree on. In his engaging book What Is Life? British Nobel Prize winner in Physiology or Medicine Paul Nurse provided insight into the debate and developed three guiding principles to define life: (1) life has the ability to evolve through natural selection; (2) life-forms are bounded, physical entities; (3) life-forms are chemical, physical, and informational machines. Nurse borrowed the title from an earlier, fascinating book by the Austrian Nobel Prize-winning physicist Erwin Schrödinger from 1994 titled What is Life? In it, Schrödinger described the physical aspects a living cell must have, which served as inspiration for American biologist James Watson and British physicist Francis Crick, who discovered the structure of DNA later in 1953.

“NASA uses a similar definition in its search for life in the cosmos: ‘Life is a self-sustaining chemical system capable of Darwinian evolution.’ But a lively discussion continues as to how to best define what life is and how to find it elsewhere.

“Cells are the smallest structural unit that can function independently. Think of a cell as a tiny chemical reactor surrounded by a membrane and containing a library of genetic information. Some of the simplest organisms alive today, tiny archaea, pack everything they need for growth, reproduction, and evolution into a single cell. Simple organic compounds can form naturally in conditions that might have been found on early Earth. In 1952, two American scientists at the University of Chicago, Stanley Miller and Harold Urey, demonstrated that the energy from lightning could have created organic molecules on a young Earth. To prove this, they filled a glass container with carbon dioxide, methane, and water vapor — the proposed components of Earth’s ancient atmosphere — and ran a spark through the gas mixture, mimicking. Lightning. This experiment resulted in brown organic material on the container’s inner walls, and the results have been replicated in many laboratories all over the world, including at Cornell by Carl Sagan.

“The energy needed for prebiotic chemical reactions would have been readily available on a young Earth – high-energy ultraviolet radiation battered the surface, heat from volcanoes drenched the surroundings, and lightning cut through the early atmosphere. Is this how life got started on Earth, out of organic material produced by lightning storms? We don’t have the answer yet. And there are other, more surprising places where scientists have found organic material. The meteorites that delivered most of Earth’s water— carbonaceous chondrites— also contain organics; a large diversity of organic molecules, like amino acids, sugars, and fatty acids, traveled through space on these ancient messengers.

“Before I started searching for life in the cosmos, I just assumed scientists knew how it started on Earth. We don’t! This fundamental question is still an area of active research. But we have figured out some of what it needs. First, life needs water. Second, it needs a solid surface where chemicals can stick together and form bonds and structures. These two requirements could be found on the rocky bottom of a pond or on the ocean floor or maybe even on the icy bottom of a puddle on an ice sheet.

“We still don’t know how the mixture of elements assembled into self-replicating molecules in which tiny variations allowed for improvement over time, leading to all the living world around us. The environment needed to provide energy and the right chemical conditions for these molecules to become encapsulated in a membrane and make a cell. That is how life escaped the ever-diluting power of the oceans and began billions of years of exploration to conquer the world.

Lithified stromatolites on the shores of Lake Thetis, Western Australia. Archean stromatolites are the first direct fossil traces of life on Earth.

“When you mix the chemicals for life in water, you need a way to concentrate them so they can start assembling the building blocks of life: RNA and DNA strands and a cell-like structure that can contain them. Scientists are making significant strides in their research on how life got started on Earth. But there is one fundamental problem: scientists can’t make life in the lab yet. For now, such a feat belongs to the imagination of visionary writers like Mary Shelley and her creation, Dr. Frankenstein. There are many reasons why making life in the lab is incredibly challenging. How do you set up the experiment (at what temperature, in salty or fresh water)? How long do you need to wait for life to get started (ten minutes, a year, ten thousand years, one million years)?

“We know roughly what the Earth’s surface was like about three and a half billion years ago— that’s when rocks recorded fossils of the first known life-forms. While we know that Earth’s surface was warm and covered with liquid water, we don’t know if these conditions were what life needed to get started. Maybe life started before a record of it became written in stone. And we don’t know if it began nearly everywhere at the same time or if it started in a tiny niche and spread over the whole globe. Life could have begun on the bottom of the ocean in places called white and black smokers, where hot water gushes out of the ocean floor into a freezing, deep part of the ocean under high pressure. Tube-like networks close to such smokers could have provided the surface for pre-cell like structures to form. Sharp temperature differences can concentrate chemicals and might have been enough to create pre-cell-like and pre-RONA-strand-like structures. If life started like that, it would not have known about sunlight or air. It would not have cared whether the planet’s surface was frozen solid or nice and warm. Is the bottom of the ocean where life on Earth started?”