What is Universal Common Descent?

Hi Ben
I think discussing evidence is fine but first you really need a hypothesis that the evidence is supporting. We are discussing universal common descent but have not yet come up with a mutually agreed upon definition.


Can you provide 2 or 3 examples of Un-Acceptable definitions of Universal Common Descent? How hard can it be to devise one?

For example, here’s the one provided in Wikipedia - in paraphrased form:

“Common Descent is the position that all of Earth’s life, shares a most recent common ancestor. There is evidence of common descent that all life on Earth is descended from the L.ast U.niversal C.ommon A.ncestor (LUCA).”

Naturally, this definition can become complicated by allowing for new life to splash down from deep space … or if you believe far back in the days of protists or viral/bacterial forms of life, there was an “A” strain and a “B” strain that emerged independent of each other. And then this triggers an arm-wrestling match about whether “A” and “B” ever co-mingled and so forth.

But to understand how hard it is to create a definition I think will require some examples of definitions that have been Rejected!

George Brooks

Hi George
Does this mean that all life on earth started from a single cell and became all the diversity we see by cell division and eventually sexual reproduction?

@Billcole, while it is difficult to make universal statements … because there is always someone with a different idea … but I would say Yes (!!) that’s what this means to many people who adhere to Universal Common Descent!

And you say?

I say this is an extraordinary claim given what we know about the sequence dependence of DNA. How in the world are new unique proteins going to form simply by cell division and sexual reproduction let alone control elements like micro RNA’s.

For that particular concern, I think it would be worthwhile to come to some understanding about the mechanisms of genetic change, how proteins may evolve and how novel sequences arise. Considering what we know about the ‘sequence dependence of DNA’ I don’t see that posing excessively high barriers to evolution.

Sorry, I’m not following you. Now you need a hypothesis to discuss the evidence, but you didn’t need one to make your sweeping statements about the evidence?

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Where do you get the idea that the first life would be cellular?

Why doesn’t diversity come from mutation, recombination, and selection?

Why would the common ancestor have DNA?

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Okay… you have lots of ideas packed into your objection. Let me sort through them.

  1. The more DNA molecules aggregate, the more chances there are for one change to have unexpected effects.
  2. If the Replication of DNA or RNA was perfect, there would be very little evolution.
  3. The Replication of DNA and RNA is fairly reliable, but it is hardly perfect. That’s why there is such a thing as cancers.
  4. BioLogos does not require a scenario where God is prohibited from being involved in the grooming of DNA and/or RNA. If some Christians think God is able to take a handful of common chemicals found in mud or clay and make a human, then certainly he could do the same with even more relevant and specific chemical compounds.

Once the hard part is over, and God has finished fashioning a pretty darn complicated molecule like DNA … then the imperfections in replications pretty much guarantee that mutations will arise. And since geologists tell us the Earth (and the entire Universe) is billions of years old … there is plenty of time for these mutations to accumulate!


APPENDIX: God’s Recipe for Humans?
Statistics vary, but if a man’s body is 66% water, then if Adam weighted 185 pounds, when God made Adam out of mud or dust, God would have had to mix about 122 pounds of water (which is about 15 1/4 gallons) with a ratio of other materials much like this formula for “Brick Clay”: Excluding about 5 pounds of water, which should already be counted in the 122 pounds, we would need the following minerals to make up about 57.5 pounds of non-water volume: 20 pounds of silica (sand), 16 pounds of alumina, 9 pounds of quartz, 3 pounds of calcium carbonate, 2 pounds of feldspar, 1 pound of calcium sulfate, 1 pound of ferric oxide (common rust, for the blood?), and a little less than half a pound each of magnesia and alakali.

Hi Argon
I agree the mechanisms are key. I think that it is exceeding unlikely that a stochastic process is capable of generating even one protein that must bind with a pre existing protein. The sequence space is too large and specific sequences must be found so protein A will successfully bind with protein B. After studying the Dawkins Weasel program, I am certain that in order to find a specific sequence you need to know the target.

Thanks for the thoughtful response and helping define UCD. I think our initial definition needs to be modified to match the challenges in cellular biology. My conclusion is that evolution of major innovative transitions requires clear complex engineering. James Shapiro has a theory that the cells can engineer new function. I think some limited engineering is going on especially during the cell cycle and DNA repair. I am a little skeptical this explains evolution because of the massive engineering to build a spliceosome or build a circulatory system.


It sounds like you are trying to prove Evolution is wrong because it cannot happen without God.
So are you a supporter of BioLogos? Or do you oppose BioLogos?

The people I speak with who support BioLogos think God makes things possible. Do you think so?


Hi George
I am new to BioLogos but have found the discussion to be very considerate and helpful.

I have studied the science of the ID community and appreciate their work but find that their conclusion is limited.

I have really enjoyed my discussions with Joshua in the past. Although we don’t always agree, I find his thinking to be interesting and challenging. I believe the theory of evolution as presented in the textbooks is ahead of the science.

IMHO this should be cleaned up and not used as an ideological tool in the classroom. I believe God is the creator of the universe and evolution is not a hinderance to my belief. I believe the theory should be rigorously tested for scientific validity which includes questioning 150 year old inferences like UCD.


Evolutionary theory, and its corollary of Universal Common Descent, has been the stimulus for tremendous advancements in medicine, history, and pure sciences of biology, chemistry, and bio-physics.

Proposals are made … and more and more frequently, some corner of the Academic community is able to provide an Evolutionarily-linked theory to close the loop and inspire yet another round of proposals.

So nobody is going to listen much to an idea that we should slow down on the theorizing.

As long as God is on Humanity’s side, I just don’t see a down-side to the BioLogos mission.
I couldn’t tell from your comments whether you agreed with this position or not. I hope you do.


Universal Common Descent (UCD) is NOT a strict requirement of mainstream science or evolution. This is just one hypothesis that is subject to revision.

Scientists regularly consider the possibility that life arose multiple times on earth.

Also remember that many scientists are hopeful we will find life in the far reaches of outerspace (e.g. SETI). Whether this hope is well placed is a debate for another day, but the important thing to remember here is that any life outside our solar system would almost certainly be from a separate origin event. We would not share common ancestry with them.

So while we regularly use the terms Universal Common Descent (UCD) and Last Common Universal Ancestor (LUCA), this is just one hypothesis among many. Alternate hypothesis are considered all the time by some scientists. Now, there is some evidence (certainly not definitive) that all extant (still living and not extinct) life on earth shares common ancestry. But given that we do not know how important convergence is in abiogenesis, it is not clear how to assess and interpret this evidence.

In conversations among Christians, the correct place to focus is the common ancestry of humans and the great apes (gorrillas, orangatangs, and chimps). The evidence here is overwhelming for common descent (genetic data, intermediate fossils, etc.), and this also where most of theological discomfort arises. The focus should be here. If we resolve/understand this evidence for human evolution in light of our theology, the rest is just a side issue.

Frankly, most people who argue against common descent just bring up UCD as a red herring. It is an excuse to argue about foggy details of microbial evolution. But who cares? Even if we grant that evolution just happens within “kinds,” we are left with the same question of human evolution. Looking at the evidence (genetics and fossils) it really looks like we are the same “kind” as the great apes.


Vertebrates do this day in and day out, without any difficulty - they use a stochastic, randomized process to make antibodies, which are proteins that bind with high affinity to a preexisting protein or other antigen. The vastness of sequence space does not impede this process in any way.



Excellent clarity !!

This in spades. If God designed us directly, He has quite a flair for irony.


That is a non sequiter and an unfortunate misrepresentation of the science which will fool many people. The fact that the immune system routinely solves difficult binding problems is irrelevant to Bill’s point. Unfortunately evolutionists have a long history of making such inappropriate claims, where the capabilities of organisms is somehow cast as a solution to, or type of, evolution. Another prevalent example of this these days is epigenetics, and the casting of it as a new type of evolution, which it isn’t.