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The common ancestor of humans and modern bacteria was most likely a prehistoric bacterium. Does this mean that humans are "more evolved" than today's bacteria?

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    $\begingroup$ To be precise, the last common unified ancestor was likely closer related to archaea than to bacteria: doi.org/10.1038%2FNMICROBIOL.2016.116 $\endgroup$
    – Thawn
    Mar 13, 2018 at 14:19
  • $\begingroup$ @NoOne that's an impressive misunderstanding of pretty much everything in biology you've got there. $\endgroup$
    – arboviral
    Mar 13, 2018 at 14:30
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    $\begingroup$ "More evolved" doesn't mean anything in biology. $\endgroup$
    – John
    Feb 3, 2019 at 4:33

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This is a standard question. The whole problem boils down to what you mean by "more evolved"? Below I am answering the question considering a number of possible definitions. If you have other ("clearly defined") definitions in mind, let us know.

Are [humans] "more evolved" than present-day bacteria?

If "more evolved" == "more evolution time"

Then, both bacteria and humans are exactly as evolved!

As all life on earth shares a common ancestor, all currently living creature has evolved for exactly the same amount of time (about 3.8 billion years).

If "more evolved" == "more generations"

Then, bacteria are more evolved.

Bacteria typically have a much shorter generation time than humans do, so with this metric, bacteria would be more evolved.

If "more evolved" == "more fixed mutations since the MRCA"

Then, it is a little more complicated and may depend upon which bacteria lineage you are considering.

If the genome-wide mutation rate is $\mu$ and the haploid population size is $N$, then there are $N\mu$ new mutations every generation. Given that each one of these mutations has a probability of fixing (fixing = reaching a frequency of 1 in the population) is $\frac{1}{N}$, the rate at which mutations fix is $\mu N \frac{1}{N} = \mu$ at each generation. If a species has $g$ generations per year, then over $t$ years, $\mu g t$ mutations would have accumulated.

The genome of bacteria is typically much smaller than the one of humans, but their mutation rate per nucleotide is much higher. Overall, the genome-wide mutation rate is rather higher in humans but that might well differ from species to species. However, again, bacteria have had more generations per year than humans and therefore bacteria species likely have fixed more mutations than humans have. So from this metric, bacteria are rather more evolved than humans.

If "more evolved" == "able to resist higher acceleration"

Then, bacteria are more evolved although that might among bacteria lineages

Some bacteria have been grown under 400,000 g (National Geographic), while a typical human can only tolerate about 5 g and for only a short period of time!

If "more evolved == more impact on the environment"

The term impact is a little hard to define but I think it is of interest to highlight that, even here, bacteria would probably win because cyanobacteria caused the great oxygenation event which is probably the largest planet-wide environmental change caused by biological organisms.

(thanks @Thawn for this definition he gave in the comments)

If "more evolved" == "having more lungs"

Then, humans are more evolved.

Bacteria have no lungs, humans have two.

If "more evolved" == "bigger genomes"

Then, humans are more evolved. Note also that corn would be more evolved than humans with this metric!

If "more evolved" == "higher fraction of the genome that is encoding for proteins"

Then, bacteria are more evolved.

If "more evolved" == "more protein coding sequences"

Then, humans are more evolved.

If "more evolved" == "more phenotypically different from the MRCA"

You would have to list specific phenotypes of interest. So, here the term "more evolved" still remained a little undefined.

Also, we do not have much information about the MRCA of these two highly distantly related lineages. The question could be more easily answered for more closely related lineages. For example, in mammals, if you consider the following traits and always ask what species is (likely) most different from the MRCA of all mammals

  • body mass -> blue whale
  • brain mass -> sperm whale (yes, sperm whales have a very big brain)
  • lips shape -> platypus (they have a beak)
  • time taken to type Hello\nWorld on a computer keyboard -> humans

Related posts

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  • $\begingroup$ +1, wouldn't you reckon this question to be a duplicate, as you state in your answer? $\endgroup$
    – AliceD ♦
    Mar 10, 2018 at 21:35
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    $\begingroup$ @AliceD Good point. I posted two related posts. The post 'What is meant in biology by the term “evolved”?' kinda make a similar point than the one I am making here. By listing potential definitions, I am hoping to make the naive reader realize that it is not that easy to come up with a good definition that would give us a good intuition about what more evolved mean. $\endgroup$
    – Remi.b
    Mar 10, 2018 at 21:41
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    $\begingroup$ Re "genetic junk", please try to understand that there's a difference between "we don't understand what this DNA does" and "we don't understand what this DNA does, therefore it must be junk". $\endgroup$
    – jamesqf
    Mar 10, 2018 at 23:18
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    $\begingroup$ more evolved == more impact on the environment even here bacteria win because cyanobacteria likely caused the great oxygenation event which is the largest planetwide environmental change caused by biological organisms $\endgroup$
    – Thawn
    Mar 13, 2018 at 13:12
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    $\begingroup$ I think it's pretty obvious to every thinking person that the objectively correct definition of "more evolved" is "able to resist higher acceleration". (More seriously, this is a fantastic answer, thank you.) $\endgroup$
    – tparker
    Apr 23, 2018 at 19:57
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While I believe that all the definitions given by Remi.b are very valuable and have merit, I would like to add one definition which I think is closest to what is commonly understood under the term "more evolved":

"more evolved" == "more genetically complex" (Edit: more specific definition as suggested by Bryan and Remi)

To be more precise I define "more genetically complex" as:

  • more different protein-coding genes (different being important here because some plants have extremely large genomes due to polyploidy. But this is not adding complexity because polyploidy means that they have many copies of the same genes)
  • more expressed gene variants
  • more posttranslational modifications
  • more genetic control mechanisms

Under this definition, humans are clearly more evolved than bacteria:

  • Humans have close to 20.000 protein coding genes, while bacteria have somewhere between 1500 and 7500
  • Humans have RNA splicing, enabling the expression of several different protein variants from the same gene. Bacteria do not have RNA splicing.
  • Humans use a wide variety of post-translational modifications of proteins to tune protein activity and for signalling cascades. This point is not as clear as the points above because the entire biological domain of bacteria has developed many post-translational modifications, and the exact number of modifications used in the entire bacterial domain is much less well understood than the post-translational modifications in humans. However, it is safe to say that each individual species of bacteria uses much fewer post-translational modifications than the human species (and comparing a single species to an entire domain would be quite unfair).
  • Humans use a wide variety of very complex interlinked genetic control mechanisms which span from expression control by cell signaling via expression control and gene silencing during embryonic development to heritable epigenetics (Lamarck was not that wrong after all). Like the post-translational modifications this is a very active field of research and more and more expression control mechanisms are being discovered both for bacteria and humans all the time. But again, there is no argument that humans have a more wide variety of genetic control mechanisms than any individual bacterial species.
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    $\begingroup$ I think it's quite a stretch to say "more evolved" == "more complex". I mean, certainly you an assert this just due to flexibility of language, but there's zero reason to throw the word "evolved" in there: If you mean more complex, just say more complex (and then get ready to defend your definition of complexity...). Although over time evolution will produce some more complex organisms than started, evolution need not result in higher complexity in all cases. $\endgroup$
    – Bryan Krause ♦
    Mar 13, 2018 at 19:51
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    $\begingroup$ I think it is a good add +1. I would just note that definition "evolved" based on "complex" and then defining complex is a little of a work around that may lead a naive reader to just retain that "human are more complex" without understanding what this means. I would rather have 4 categories and drop the term complex. I am not sure my criticism is the same as @Bryan's $\endgroup$
    – Remi.b
    Mar 13, 2018 at 20:50
  • $\begingroup$ @BryanKrause; @Remi.b I changed the definition to "more genetically complex". This is more specific and hopefully more clear. What do you think? $\endgroup$
    – Thawn
    Mar 14, 2018 at 12:10

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