Evolution

Exploring the Marvels of Evolution: From Single Cell to Complex Life

Today we are talking about evolution, which is defined by the Oxford dictionary as, “the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.” To be more precise, we will talk about the process that started with a planet without any life at all (abiotic earth), and ended with the diverse range of species found on earth today.

Within this important (and fascinating) topic, we will focus on two related sub-topics: (1) the history of how life has evolved; and (2) the history of how researchers investigated and tried to understand evolution, and the process by which life has evolved. We will end this article by reviewing the current scientific understanding of evolution, and what this understanding might mean for the future of our species.

History of How Life Has Evolved

So, how did life evolve on earth?

The first step of evolution, which we can define as the start of life on earth, took quite some time. Our planet is approximately 4.54 billion years old, and researchers estimate that the first living cell developed only 3.8 billion years ago. This means that there was no life on earth for approximately 740,000 million years.

 

Chemical Evolution

So what was going on during this time? A different, but related process, called chemical evolution. Chemical evolution refers to building up of complexity in molecules, from single atoms and small molecules to complex biological molecules. In order for life to start, there needs to be an availability of relatively complex molecules that make up a cell membrane, or the barrier between cells and their external environments. The molecules also need to have very specific three-dimensional structures (a characteristic called “chirality”) so that they can form cell membranes and the functional parts of a cell.

Getting to this level of molecular complexity was a prolonged process on early Earth, but a necessary prerequisite for life to begin.

 

The Beginning of Life

Once the molecules were the appropriate level of complexity, life was able to begin. What do we mean by “life”? Most scientists define life as a self-contained structure, like a cell, that is able to replicate itself. The early cells were probably made of a cell membrane, or a barrier between the cells and the environment, which was made of lipid molecules, as well as some kind of oligonucleotide found inside the cell that was able to replicate.

What does this mean? The lipid molecules that made up the early cell membranes (and are key components of current cell membranes as well) are the same kinds of molecules found in fats that we eat – oil, butter, and other delicious foods. The oligonucleotides found in early cells were similar in structure to DNA, which contains all of the genetic information of current cell types, but were simpler in structure in early cells.

 

The First Multi-Cellular Organisms

From this first cell, it took a lot more time for multi-cellular organisms to evolve. This is because organisms that are made of only a single cell can very easily figure out how to get nutrients from their environment and how to get rid of waste products – they just pass the stuff through the cell membrane! In a multicellular organism, or living thing composed of many cells, the organism needs to figure out a way to transport materials between cells: so that the cells on the outside that take in nutrients can get those nutrients to interior cells, and the waste products generated by interior cells can be excreted by the exterior cells into the environment.

When did this transition to multicellular organisms occur? Probably about 1.7 billion years ago, which means that life on earth was completely unicellular, or made up of single-celled organisms, for more than 2 billion years! What helped evolution to develop multi-cellular organisms was probably the evolution of plants (or precursors to plants), which generated oxygen as a waste product of their photosynthesis. Having oxygen in the environment provided a much more efficient energy source, which meant that more complex organisms with multiple cells were able to obtain the energy that they needed.

 

Kingdoms

Notably, both plants and animals transitioned from unicellular to multicellular organisms around this time, and then each category of organisms (called “kingdoms”) evolved further. There are other kingdoms, or categories of organisms, that never evolved into multicellular organisms, such as bacteria, protists, and archaea, and only contain unicellular organisms. Finally, the kingdom of fungi (plural of fungus) is unique in that it does have multicellular organisms, but also still has unicellular organisms that exist even today. For example, mold is a multi-cellular fungus, whereas yeast (the kind you use in bread) is unicellular.

 

Humans

From the start of multicellular organisms, it took quite a while until the first humans evolved. First, the multicellular organisms differentiated further: multicellular fungi, multicellular plants, and multicellular animals. Within each kingdom, organisms started to differentiate into different sub-categories, called “phyla” (singular: “phylum”). The animal kingdom differentiated into animals with a backbone-like structure (phylum of chordata) and those without this structure. The phylum of chordata differentiated further into different “classes” of organisms, including the class of mammals. This class of mammals differentiated even further, until approximately 350,000 – 260,000 years ago, the first Homo sapiens, or humans, appeared.

 

Carbon Dating

You may be wondering how scientists know this much information about the evolution of life, particularly when it comes to events that happened many millions (and billions!) of years ago. Much of this information comes from fossils, which scientists discover at various locations around earth, and it also comes from the ability of scientists to study the atoms and molecules found in those fossils. More specifically, each atom in the fossils exists naturally as more than one isotope (or highly related structure with slightly different mass), and by measuring the ratios of the different isotopes, scientists can figure out how old the fossils are. This process, which is often called “carbon dating,” is used as a key component of studying evolution, and of figuring out the approximate timelines of how molecules, cells, and people evolved.

In the 1800s and 1900s, however, scientists didn’t have the ability to measure isotope ratios. They were lacking the ability to measure a lot of things, in fact, which made it more difficult for them to understand the process of evolution. What could they do? Mostly, the scientists of the time were able to observe living organisms, and use their observations to hypothesize about the process of evolution.

This brings us to the second half of the blog post, which is a review of what scientists thought about evolution.

 

History of Scientists’ Hypotheses About Evolution

For a long time, people didn’t even think about evolution. They thought that the world had been created by a supreme being, and that this being had decided to create the wide variety of organisms that everyone could see. This kind of approach almost always has humans as the center, or main point, of the creation.

 

Jean-Baptiste Lamarck

One of the first scientists who investigated evolution in detail was Jean-Baptiste Lamarck, who lived from 1744-1829. He believed that organisms could pass on certain characteristics that they developed during their lifetime, a theory termed the “inheritance of acquired characteristics.” In this kind of theory, somebody who worked with their right arm and developed strong muscles on that side of their body would have offspring who were born with more strength on their right sides.

More broadly, Lamarck thought that certain traits would be passed on through use, while traits and body parts that were not used would not be passed on. He explained that the reason why giraffes have long necks is because their ancestors had to stretch their necks to reach the high leaves; those giraffes that were successful in reaching those leaves were more likely to survive and pass on their long necks to their offspring.

While some parts of Lamarck’s theory have proven be true, including the understanding that certain features are inherited and the idea that organisms build up in complexity, most of his theories are no longer considered to be valid.

 

Charles Darwin

Charles Darwin, who lived around the same time as Lamarck (1809-1882), developed different ideas of evolution. His ideas were developed and fine-tuned by observing finches, species of birds that are found on the Galapagos islands. During a voyage to these islands Darwin noticed that each of the islands had birds with slightly different features. Upon returning from his voyage, Darwin used these and other observations to write a book, On the Origin of Species by Means of Natural Selection, more commonly referred to as On the Origin of Species.

In this book, Darwin laid out many of his hypotheses about evolution, including the main idea of natural selection. This idea states that evolution selects for those members of a species who have beneficial traits, and deselects for those who have harmful traits. In other words, giraffes whose necks were slightly longer than average were more likely to survive than giraffes whose necks were shorter than average. Over several generations, this means that the average neck length of a population of giraffes is likely to increase.

There are many other ideas that are either explicitly or implicitly stated in Darwin’s work, including the idea that there is a competition for natural resources that means that all members of a species cannot survive; that overall species have increased in complexity over time; and that certain traits are inherited, or passed down from parents to their offspring.

 

Modern-Day Scientists

What do modern-day scientists think of Darwin’s theories? Well, a good part of what he hypothesized turns out to be fairly accurate, which is amazing considering that he didn’t have any knowledge of genetics. The science of genetics, which is generally defined to include the study of how genes encode for certain traits, was popularized by Johann Mendel, a botanist who studied pea plants. Although Mendel lived around the same time as Darwin and Lamarck (1822-1884), his work on genetics did not become well-known until after he died, in 1900. Without this knowledge, Darwin referred to a “mechanism of inheritance,” which he postulated even without knowledge of genetics or DNA. Darwin also postulated details of this mechanism, termed ‘pangenesis,’ in which ‘gemmules,’ or seeds of cells, were provided by each parent to create their offspring. We know today that this is not how genetics works.

Where does all of this information leave us? Well, you may be interested to know that scientists are pretty sure that humans are still evolving. What evidence do we have for this?

 

Are Humans Still Evolving?

People all used to have wisdom teeth, a third set of teeth that helped them chew food into smaller pieces. As humans evolved to use utensils rather than just their teeth (and hands), the percentage of the population born without wisdom teeth has increased. Today, approximately 35% of the population is born without these teeth, and scientists expect this percentage to rise in future years.

You may also be interested to know that human brains are shrinking, which scientists think is also part of evolution. Not because we are getting dumber (or at least not most of us), but because our brains are evolving to be more efficient at what they do.

People are also getting taller on average. Is this due to evolution as well? Scientists say no! Rather, this kind of change is more likely due to improved nutrition, which means that people can reach the maximum height allowable by their genetics.

Overall, expect that people and all organisms will keep evolving, and be on the lookout for the next round of new evolutionary changes!

 

Author: Mindy Levine, PhD

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