Answers To One Reader’s Questions About Common Ancestry

I get email, albeit rarely.  Last week I received an email from one reader listing four questions about common ancestry.  Since I see questions like these pretty often — either from students or on evolution/creation comment threads — I thought I’d post the questions and their answers for all to see.

Dear [Me],
I am assuming that you are the person who writes about evolution in
‘darwinbookcats’.  I hope so.  While talking to school and college students about evolution we run into a set of questions that are often asked by them.  I list them in the hope that your explanation can be helpful in discussing these issues.

With all good wishes for your continued contributions.
Sincerely,
[Reader]

1. If all groups of living organisms evolved from ‘common ancestors’ there must be millions of common ancestors. What are some of the common ancestors (of any group) living today?

Before I answer this, let me first define what a common ancestor is and where it comes from.  A common ancestor is a historical population that, through the process of speciation (or splitting) gave rise to two or more groups alive today.  The concept of a common ancestor comes from the evolutionary principle of common descent — the inference that all living species evolved, through descent with modification, for some ancestral population.  We can illustrate this concept with the image from Baum’s “Tree-Thinking Challenge” that I used in a previous post on phylogenies and evolution.  

In the diagram to the left, the points labeled y and x both denote common ancestors: y is the common ancestor of all the organisms shown (vertebrates, or animals with backbones), and x is the common ancestor of amphibians, reptiles, and mammals (tetrapods, or vertebrates with four limbs).  There are two important points to remember.

First, all species have multiple common ancestors depending on which two species you compare.  For example, the point directly below humans and mice would be the common ancestor of the mammals shown, while the point below that would be the common ancestor of humans, mice, and lizards (amniotes, or tetrapods with shelled eggs).  Both are common ancestors of humans, just for different groups of species.  Given this fact, you are correct: there must have been millions of common ancestors, since we can identify numerous common ancestors for any two species you want to compare.

Second,  common ancestors result from speciation events, in which two new species form from the splitting of one ancestral group.  Speciation (also called cladogenesis) is the main pattern of evolution responsible for the diversity of life today.  A second, but distinct, pattern of evolutionary diversification is anagenesis.  In anagenesis, an entire ancestral population accumulates enough genetic and phenotypic changes that it becomes a new species from its original form.  However,  since the entire population evolved, the common ancestral population now exists in this new form; therefore, in the case of anagenesis, the ancestral population goes extinct.

There is a lot of biological evidence for common ancestry, including the use of DNA and a common genetic code among all living organisms.  Now that we know what a common ancestor is and how they form, I can answer what some of these common ancestors are in your last question below.

2. Did Homo sapiens evolve from another species (say H. hiedelberghensis) or did they both have a common ancestor? And what happened to that common ancestor?

This is a very good question.  H. heidelbergensis is an extinct Homo species that first arose 400,000 – 600,000 years ago.  It is likely that H. heidelbergensis relates to modern humans by cladogensis: H. heidelbergensis is likely the ancestral species that gave rise to both modern humans and our closest relative, Neanderthals (H. neanderthalensis) (see the figure from Klein (2009) below)

Evidence from the fossil record indicates that H. heidelbergensis continued to inhabit African and Europe after the initial divergence that gave rise to Neanderthals and then modern humans; however they went extinct around 100,000 years ago.  The extinction of our ancestral H. heidelbergensis populations likely occurred as a result of competition with its better adapted descendent species, H. neanderthalensis and H. sapiens.

3. Why is evolution so unidirectional? i.e. why are no seed plants evolving from the thousands of ferns living today? Why are no vascular plants evolving from bryophytes living today? Why all the thousands of species of birds remain birds and not evolving into something else from among the bird group? Why human-like or other bipedal or similar creatures not evolving from the primates at least some of which can act as common ancestors?

If by unidirectional you mean ‘moving from simple to more complex’, this answer is that evolution is not.  There are many examples of new species that are less complex than ancestral ones, including blind cave organisms and parasites.  The concept of a “ladder” of evolution that always leads to more complex species is a human construct.  In the figure above, look at the tree on the right: it is the same as that on the left, but with the nodes turned.  From this scenario, you can tell that evolution may not necessarily be a ladder.

For the other part of your question, the existing species of ferns, bryophytes, and birds are evolving, albeit into new species of ferns, bryophytes, and birds.  We will never be able to directly observe their evolution into new and different groups because there are simply too many differences to evolve at once.  For example, the process of cladogenesis that lead to the evolution of ferns and bryophytes took hundreds of millions of years and affected numerous genetic and physical traits, including their life cycle.  It’s simply too big a jump to see in a human lifetime of 70 years.  Unfortunately, the type of evolution that we can directly observe is generally the small-scale, a-few-traits-at-a-time, microevolutionary stuff.  Microevolution is the same as macroevolution, just over a shorter timescale.

If we could revisit earth in another 300 million years (and take DNA samples of species now and then), we would find that the new species of plants and animals alive then are the descendents of the ferns, bryophytes, and birds alive today.

4. Why we always talk about common ancestors in the past? Why not there be active common ancestors now among the 30 – 100 million estimated species?

As mentioned in the case of anagenesis and the example of Homo heidelbergensis, the common ancestor of most of the living species you might compare is extinct.  However, for cases of recent speciation (within the last 100,000 years), we can point to specific common ancestors, many of which are alive today.

One easy example is the case of modern dogs (technically all one species).  Modern dogs evolved through artificial selection from the grey wolf (Canis lupus) starting approximately 15,000 years ago.  All modern dog breeds are the result of subsequent artificial selection from this domesticated stock.  Thus, the grey wolf is the common ancestor of all modern dogs.

Another example is a group of tiny lake fish called sticklebacks.  All lake sticklebacks evolved from oceanic stickleback populations over the last 15,000 years.  Despite their evolution of new color, armor, pelvic fin, and life history traits, we can still view their ancestral form in existing oceanic populations.

Good questions, and thanks again.  I hope these answers help, and keep those questions coming.

References:

Klein, R. G. (2009) Darwin and the recent African origin of modern humans. PNAS 106: 16007-16009. DOI: 10.1073/pnas.0908719106.

Baum, D.A., Smith, S.D., Donovan S.S.S. (2005) The Tree-Thinking Challenge. Science 310: 979-980. DOI: 10.1126/science.1117727.

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