With other members of the Liu lab and BME department at UCI, I'm participating in a program called Rocket Science Tutors that teaches hands-on science classes after school at low-income middle schools in Orange County (mostly in Santa Ana).

Our fearless leader Nino started an "ask a scientist" program last week where we distributed slips of paper to students, asking them to write (anonymously) their questions for us, which we promised we'd answer at the next class session.

We got one response the first week. Someone asked "if evolution is real [ed. note: danger!], how did life form on Earth and what did the first organism look like?"

It's possible we're being trolled -- but it's a really good, and really deep, question. I spent way too long researching a good answer, which I'll paste below; it made me realize that I didn't have a very good answer and I was surprised by a lot of what I found. I wanted to answer the question credibly without taking too much class time, so my writeup may be a little scant. What do you think? Is this is the right way to answer?

How did life form on Earth, and what did the first organism look like?

Print for document camera:
http://upload.wikimedia.org/wikipedia/commons/7/77/Geologic_Clock_with_events_and_periods.svg
http://upload.wikimedia.org/wikipedia/commons/5/54/Miller-Urey_experiment-en.svg

Early Earth

The Earth formed from leftovers from the formation of the sun about 4.5 billion years ago.</p>

That’s: 4,500,000,000 years
First hominids: 2,300,000 years
Modern humans: 200,000 years
Human culture: 50,000 years

Early Earth would have looked very, very different from the Earth today. For example, who knows what the atmosphere is made of today?

Oxygen is really important to us. But around the time that life must have emerged on Earth, the atmosphere was made mostly of carbon dioxide, nitrogen, and some hydrogen. There was no oxygen at all. What do we know about carbon dioxide in the atmosphere? Why are we worried about carbon dioxide today?

There are very much small amounts of carbon dioxide in the atmosphere today, and even a small increase is causing big problems for people. So you can guess that the early Earth was very, very warm. In some places the ocean would have been warmer than a hot bathtub.

Forming amino acids

The early Earth had lots of rocks and lots of water. But we’re not made of rocks and water, so some kind of chemical reaction must have happened to generate the chemical building blocks of life. For example, we’re all made of proteins, right? Does anyone know what proteins are made of?</p>

In 1952, chemists named Stanley Miller and Harold Urey at the University of Chicago conducted a very simple experiment: if they made up an early Earth atmosphere in the lab and zapped it with electricity to simulate lightning, could they make complex molecules?

They found that they could, and that it didn’t take very long. Within a day, the water was pink, indicating that reactions were happening. When they looked at the mixture at the end of two weeks, they found amino acids, sugars, and other molecules essential for life.

Stanley Miller became a professor at UC San Diego and the original experimental apparatus is at the Scripps Oceanography Institute in La Jolla today.

This is just one of many possible ways that the chemicals needed for life could have arisen on Earth. There are many other theories.

Last common ancestor

Let me answer another question for a second: what did our first evolutionary ancestor look like?</p>

The idea that all life on Earth evolved from a single kind of life is called the “last universal ancestor” theory. Charles Darwin himself first proposed it.

From what we know by comparing different kinds of life that are alive today and studying the similarities and differences between their genetic codes, we can make pretty specific guesses about it.

The short answer is that it would look a lot like a modern bacterium. It would have been DNA-based, it would have made proteins, and it would have a cell membrane much like ours. It probably appeared around 3.5 billion years ago.

Origin of the first cell

But how did we get from a mix of chemicals to our first ancestor?</p>

Fossilized cell colonies and other evidence tell us pretty clearly when it happened, within the first 500 million years after the Earth was formed – compared to how long it took for mammals to evolve, life arose very quickly. Earth has had life for most of its history, even though we’re very recent.

The short answer is that we’ll never know for sure. But we have a pretty good idea that each of the individual parts of a cell could have organized themselves. If you leave amino acids in a drying puddle, they form protein-like aggregates. If you shake up a solution of lipids in water, they organize into enclosed spheroids just like cell membranes. And the early Earth was a very efficient mixer: with constant hurricane-force winds and 1,000-foot tides, all of the components could have found each other.

So that’s the story. The chemical ingredients of life started forming as soon as the Earth started to cool and the oceans appeared. Within the next 500 million years, the chemicals came together into cellular life and evolution began. Another 500 million years later, our first direct ancestors were born. And now, 3.5 billion years later, here we are today.

(Sources: mostly Wikipedia! The abiogenesis article isn't as coherent as it might be but links from it give a pretty full picture.)