Charles Darwin, in the last paragraph of the introductory chapter to his magnum opus On the Origin of Species, wrote:
No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he makes due allowance for our profound ignorance in regard to the mutual relations of all the beings which live around us. Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare?
And he was not afraid to admit the limitations of his theory, devoting a whole chapter (Chapter 6, Difficulties in Theory) to questions paired with his own best attempts to resolve them in a theory-consistent way. Darwin underscored the importance of continued exploration, stressing that understanding the origins of life and understanding life itself are essential to developing mastery of our physical domain.
So, in celebration of the 206th birthday of the man who systematically gathered evidence to shape a theory upon which remarkable scientific discoveries of today still rest, we can revel in the progress we’ve made in understanding our world, breathing in an air of optimism as we dream of how much further we can go.
Genetics: Three-Parent Babies & Photosynthesizing Sea Slugs
We are certainly nowhere near mastery of genetics, but we are getting closer to knowing how much we don’t know, which is progress in itself. While Darwin realized that species changed over time, he never reconciled this with Mendelian genetics, mostly because he was unaware of Gregor Mendel’s work with inheritance. Mendel did have a copy of Origin of Species, but perhaps didn’t make the connection between his work and Darwin’s. The two theories weren’t merged to form the foundation of evolutionary biology until the late 1920s–early ’30s.
Today, we know that most changes take place through mutations that occur from transfer of genetic material, DNA, from the parent(s) to the offspring. We even know, through phylogenetics (the study of evolutionary relationships among organisms) that the energy-producing organelles in our cells, called mitochondria, have their own separate DNA molecules of a different structure from our own nuclear DNA, suggesting that they evolved as a separate lifeform and became integrated as part of more complex organisms through endosymbiosis.
Using that knowledge, researchers have now created an in vitro reproductive process to replace a mother’s defective mitochondrial DNA (mtDNA)—which otherwise can lead to miscarriage or serious mitochondrial disease—with mtDNA from a healthy donor, leading to the controversial “three-parent baby.” On February 3 a UK bill allowing for the creation of three-parent babies was passed in the lower House of Parliament. It has yet to be formally discussed in the House of Lords. In the United States, FDA approval for researchers to study mtDNA transfer in humans is still pending.
Without the knowledge of genetics, Darwin could only formulate limited questions on species change, such as: Why is it that interspecies breeding results in sterile offspring while intraspecies breeding of parents with different traits resulted in fertile offspring? Where is the evidence of the transitional species that were beaten out by the better adapting species? And, can learned traits be passed down through evolution? (The simplest answer is yes, some can.)
Now we know the transfer of traits is even more multifaceted. Multicellular organisms primarily transfer genetic information through sexual reproduction, combining the materials of both parents in the offspring, thus any new traits from mutations and genetic combination would only occur in the next generation.
Unicellular organisms, on the other hand, most commonly reproduce asexually, creating a new copy of themselves, with new traits only a result of mutation from replication of a single strand of DNA. But, as we have learned, horizontal gene transfer, that is the transfer of genetic information from one organism to the other through means other than asexual or sexual reproduction, seems to be the primary way evolution takes place in unicellular organisms, allowing for events such as antibiotic resistance by bacteria.
This has tremendous significance for human gene therapy, a field of research into medical treatment through genetic information modification in lieu of surgery or medication, especially in light of the recent discovery that shows it’s possible for gene transfer to occur between multicellular organisms. Though scientists have known since the 1970s that the Elysia chloritica sea slug, informally a sacoglossan or green sea slug, takes chloroplasts (cellular organelles responsible for photosynthesis) from the algae it eats and that these chloroplasts function endosymbiotically within the slug, scientists have only recently found evidence of gene transfer from algae to the sea slug. Specifically, marine biologists identified an enzyme found on the sea slug’s chromosome that is vital to algal chloroplasts’ functioning and would explain why chloroplasts remain functional in sea slugs for months longer than expected after being ingested.
Macroevolution: Mapping Our Origins, Geographically and Evolutionarily
With more than 200,000 years of purported evolution from archaic humans (Homo variations) leading to the modern human (Homo sapiens), it is understandable that our own evolutionary tree still has many blanks and that there are multiple versions of our migration maps. Furthermore, during Darwin’s time, speculations on the age of the earth ranged from 6,000 years, the socially acceptable biblical earth age, to somewhere less than 100 million years old, according to estimations by physicist Lord Kelvin and later by Darwin’s astronomer son, George. So Darwin’s own considerations of evolution were confused by shorter estimates of earth’s age than he believed evolution required—a point that is in retrospect true.
It is commonly agreed that modern humans originated out of North Africa and migrated to the rest of the world in varying paths 60,000-70,000 years ago (there are competing dispersal theories), though the details are yet to be ironed out with scientists—a theory informally referred to as the recent African origin of modern humans theory or the “Out of Africa” theory.
Significant support for the “Out of Africa” theory came in 2008, when researchers from the Hebrew University of Jerusalem discovered an approximately 55,000-year-old skull in Manot Cave, Israel, a well-preserved cave accidently unearthed by a construction project. Their analysis was published in Nature in January of this year.
It is the oldest modern human specimen found outside of Africa and directly within the Levant, a geographical region roughly located where Israel, Lebanon, Syria, and Jordan are today, and where humans migrating out of Africa travelled before colonizing Eurasia.
It’s also the first physical evidence (genetic studies have already pointed to this conclusion) of coexistence and interbreeding—earlier than we had previously thought—between modern humans and the more robust Neanderthals (Homo neanderthalis), who share a common ancestor with us. This further supports the theory that the predecessors of European and Asian migrants, who have between 1-4 percent Neanderthal DNA, passed through the Levant (ostensibly, from Africa).
Most Unintuitive Evidence of Evolution?
For naysayers who still deny evolution, scientists recently discovered a type of deep-sea bacteria that has remained genetically unchanged for at least 2.3 billion years. What sounds like a source of celebration for anti-evolutionists is actually resounding proof of Darwinian evolution—strictly because of the conditions under which the evolution did not occur.
Darwin stated that there is no one rate of evolution, simply because changes within species depend on a wide number of factors, so it would not be inconsistent for species to stay the same for longer or change at slower rates.
I believe in no fixed law of development, causing all the inhabitants of a country to change abruptly, or simultaneously, or to an equal degree. The process of modification must be extremely slow. The variability of each species is quite independent of that of all others. Whether such variability be taken advantage of by natural selection, and whether the variations be accumulated to a greater or lesser amount, thus causing a greater or lesser amount of modification in the varying species, depends on many complex contingencies, on the variability being of a beneficial nature, on the power of intercrossing, on the rate of breeding, on the slowly changing physical conditions of the country, and more especially on the nature of the other inhabitants with which the varying species comes into competition. Hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change less.
In essence, the unchanged bacteria reflect what in statistics is referred to as the null hypothesis, the statement that appears to hold true when there is no statistical evidence of a relationship between variables; thus, if an organism lives in an unchanging physical or biological environment, there is no compulsion to evolve over time.
To borrow a quote from a brilliant article published in Smithsonian Magazine in 2009 on what Darwin didn’t know, Francisco Ayala, a biologist from the University of California, Irvine, said “Darwin didn’t know 99 percent of what we know, but the 1 percent he did know was the most important part.” It’s just up to us to continue filling in the blanks.
For more refreshers on Darwinian evolution, check out some of our curated resources at the International Darwin Day website.