Evolution is the only natural explanation. And it's all we need.

With the ongoing adoptions, state by state, of the Next Generation Science Standards (http://www.nextgenscience.org/), and with the familiar cries of resistance against a national level emphasis on both evolution and climate change...

http://evolutionpsa.tumblr.com/

...I thought it might be a good time to point out to everyone, new or old, big or small, nonbeliever or believer, something that's often overlooked:

Evolution is our only natural explanation for nature, life, biology, you, me, peaches, lobsters, trees. 

All other explanations are supernatural. 

All. Others. Are. Supernatural.

It's the same for explaining televisions and toasters...we have only one natural explanation: physics. 

All other explanations for toasters are supernatural.

Toasters are as supernatural as tarantulas. 

So today...just so everyone's up to snuff with our only natural explanation for all of biology, here's a brief guide. 

The Earth is old. The universe is older. 

This is what we poetically refer to as deep time. 

And over all these vast stretches of all this deep time, and across our vast continents, and underwater within our vast oceans, there's been a lot of reproduction. 

Our planet, under a UV lamp, glows like a cue ball. 

And now you know why. (source)

So because of all this reproduction all over the place, stacked up in deep time, there's been lots of lineages and, over time, changes have occurred along those lineages because offspring are rarely identical to parents or siblings. 

That’s not just because kids are a unique combination of only some of each parent's information, but kids are also different from mom and dad (or whatever spawned them) because each individual also has new information (mutations) that their parents don’t.

I do. You do. We all do. 

We're all built of brand new combinations of old parental molecules (i.e. old mutations) mixed with absolutely brand new molecules (i.e. brand new mutations). 

As in brand new to the Earth. And maybe even brand new to the universe.

So here we are then. There's been change over deep time in numerous (to be ridiculously conservative) lineages. Everything alive on Earth right now is not only unique but is the end of a unique lineage that began 4 billion years ago. 

You. Me. Babe the bacon. Frank the bean. Each of us is the end of 4 billion years of what Sagan called an "unbroken thread."

The processes of change that occur across these unbroken threads, these processes that change living and nonliving matter, unfold before our eyes during our lifetimes. Just as they've been doing so throughout Earth's history. This is what's known as uniformitarianism. It's an unfortunate term that appears to mean that things have been uniform over time but it actually means that change has been constantly occurring through time.

(link to quote source video)

Change, or evolution, has always been happening and it will always happen into the future.

That's just another way of defining time: perpetual change. 

So, one of the few rules of our universe is time goes on and that means constant change occurs and because of that constant change there is always variation.

The landscape at the beach is different moment to moment. Each member of a sea turtle lineage is different generation to generation.

There's been so much deep time and so much change during it that what we see on the planet right now is only a tiny fraction of the life and the variation that lived before today. 

Most everything in my and your lineage is long dead by today. 

Further, most lineages that have existed have ended before today, just like my neutered and spayed dogs' will certainly end with them, and like mine will end if I don't "have it all."

But humans have named it all!

We have names for just about everything alive now and that makes "humans," "apes," "monkeys," "dogs," "lobsters," "sea turtles" seem separate, disconnected from one another, naturally. Especially since most species are designated by their inability to reproduce with others--talk about separate!

But, for example, if you go far enough back in my lineage there'd be an ancestor you'd probably call an "ape" rather than a "human." 

Australopithecus afarensis reconstruction from 3.4 million year old fossil bones found in Ethiopia (source)

Or you might get creative and call her an "ape-human" but that's just our limited vocabulary based on animals we know are alive right now. Then if you go further back in my lineage, you'd arrive at a point where something would remind you more of a squirrel than a human or an ape. The same thing's happening. You're relating my ancestors, 65 million years ago, to what you're familiar with on Earth today. And that's fine, but today's just a snapshot of all Earth's history and those aren't actually squirrels like the ones in our dumpsters and freezers today. 

Every single one of those ancestors in my, and your, lineage is unique. None would look exactly like anything alive now. Just like I don't and you don't. Many of my ancestors would seem like mixes of different things alive now. Many would have traits that aren't in existence on Earth right now!! If you keep going further back in my lineage, you'd see what looks like today's reptiles, and further there'd be fishes. 

Maybe that's because there are limited ways to be alive during 4 billion years on planet Earth...

hippocampus and Hippocampus (source)

Anyway, at no point does a mother seahorse give birth to a kid reptile. At no point does an ape give birth to a human. At no point does a human give birth to a little green man species, except that a scientist, accustomed to divvying up the spectrum of variation in the world into boxes, says so.

See how it's so completely obvious where red ends and yellow begins? See where everyone, even with the same exact eyeballs, would agree that blue ends and purple begins?

Not. They would not. (source; and fun; and fun too)

Color labels are fraught with some of the same issues as our labels for life. They are discrete words that stand for fuzzy-edged concepts that arbitrarily break up a spectrum and, thus, they interfere with the debate about human origins, about when "human" begins in the fossil record. Same for "ape" beginnings or "chimpanzee" or "dog" or "plesiosaur" or anything. 

Nature is a connected spectrum of variation over space and time that we must arbitrarily cut apart so we can talk about about it and study it the only way we know how.  But that doesn't mean we forget that populations, species, genera (whatever we're labeling), like the rest of nature they belong to, are neither uniform nor discrete. 

Species are not uniform across space and time, are not discrete across time, and aren't necessarily discrete across space either. (source)

Species are not uniform across space and time, are not discrete across time, and aren't necessarily discrete across space either. 

If species were discrete across time, we'd have no reproduction. 

If species were uniform across time and space we'd have no evolution. 

So the way we're tempted to think of species as essential, as discrete and uniform, is not only inaccurate but it is antithetical to what's occurring in nature, antithetical to constant change and common ancestry of each unique individual, which is all antithetical to evolution. 

It's not just the words we use specifically to understand that sometimes inhibit our understanding (!)... we're also working against our limited scope. 

We're here for only a fraction of what's been happening in the universe and will continue to happen into the future. As a result, we don't get to witness much evolution in action. 

We can witness evolution in lineages that reproduce quickly like cane rats better than we can witness it in, say, humans and chimps. It's unfortunate but we don't get to watch evolution in long-lived species as if we're watching a film.

We can, however, reconstruct that film using the biology of living, dead, and sometimes well-preserved extinct creatures. We just have to compare and contrast their traits.

Things that are more similar genetically, anatomically, etc... are more closely related to one another than things that are less similar. That's all there is to it. 

I could probably pick your parents out of a lineup if I'd never even met them. The same sorts of cues, and methods for identifying those cues, are applied beyond our families and our species to establish the fact that dogs are more closely related to wolves than to dogfishes. Humans are more closely related to dogs than to dogfishes. 

And we can reconstruct these family trees--populated by people, dogs, wolves, dogfishes--far beyond our own parents and grandparents, and back into deep time. 

T. Ryan Gregory's 2008 paper "Understanding Evolutionary Trees"

And we can test those reconstructions with help from the fossil record. 

But our reconstruction of the Tree of Life is not a perfect reconstruction because fossils are usually restricted to preserving only bits and pieces of the biologies of organisms--parts like bones and teeth. 

Plus, we don't have fossils of every single thing that ever lived. That would be impossible. Dead bodies have to be recycled into living bodies for there to be living bodies. 

link

If the fossil record was complete, we probably wouldn't be here to study it.

And even if we are lucky enough to find a fossil that was lucky enough to preserve, that's just that one animal struck dead at one little moment in its lifetime, at one little flash in the entire 14 billion years since the big bang. 

And we often struggle to link relatedness up, and reconstruct change over time in lineages in the fossil record because we rarely get more than two generations in the same deposit.  And when we do, one is usually teeny tiny compared to the adult. 

Pregnant plesiosaur (Source: Robin O'Keefe)

The fossil record is a great source of evidence for evolutionary history and that it's necessarily limited is okay. As discussed above, we actually don't even need fossils to support evolution over a supernatural explanation, because living organisms provide enough evidence.  

We look at chimpanzees and bonobos and see they share more with us and we with them than anything else on Earth. When we line up our molecules, we find the same thing, just as we  predict. These relationships we deduce among living things don't require any fossil evidence to support, but fossils do help to make sure, no doubt about that. And fossils show us all the weird creatures that had to live and die first in order for all of us to be here now.

So, fossils or not, evolution explains all the similarities we have with everything else alive on Earth. That's our common ancestry at play.

And evolution explains all the differences too. That's mutation at play. Mutations are always occurring. They are part of the uniformitarianism that describes the constant state of change, of flux, of all of history. Change over time is also due, at the population level, to differential reproduction. Some individuals have more offspring than others. Therefore, future generations are always comprised of a different biological landscape, both with higher and lower proportions of some variation compared to their parents' generation, and always with some brand new mutations thrown in there as well. 

So...so far so good. We've got deep time, change over deep time in related lineages from common ancestors, like family history writ large. Some ancestors are preserved as fossils, most are not and many of those that are have not been discovered yet. Most evolutionary history is unobservable because of the inconvenient impossibility of time travel. Similarities among organisms reflect shared heritage. Differences and distance reflect constant change that occurs via mutations and constant change over the generations due to the fact that some individuals have fewer offspring than others, leaving their mutations, their traits, in lower frequencies in the next generation. 

Darwin's term "selection" describes some of this differential reproduction among individuals. Some variation dies, some variation does not reproduce, some variation enhances reproductive output and when it does, that variation shows up in greater proportion in future generations... as long as it continues to enhance the reproduction of those with it...a condition that is usually heavily determined by the environment where all this reproducing is taking place! 

Like mutation, differential reproduction is always occurring. That means that selection is always, on some level, occurring. Some traits, like a new mutation that prevents growth of the ovaries, would prevent that woman from passing that on. Selection can also describe how mutations or variation proliferates in future generations if it causes those individuals to contribute more of their variation, their mutations, to the future gene pool than others.

Selection describes how a baby that could fit through the birth canal and, with those adequate birth canal genes, can grow up to have an adequate birth canal for fitting her own offspring through. It's that simple. And babies with genes for adequate birth canals are much more common in a population than otherwise.  Fitness (a.k.a. reproductive success or reproductive output) is such a fitting term when it's truly about fit!

But saying that "differential reproduction is always occurring," also means that drift, which is differential reproduction due to chance and differential passing of this or that gene into future generations due to chance, is always occurring too.  

And in spite of constant mutation and constant drift... that is perpetual chance change...we've managed wonderfully for 4 billion years.

It's far too common to read descriptions of evolution as if selection is the only or the main process for change and they're not telling the whole story. 

Change is perpetual underneath all of that selection. Selection, as a result, is weaker than we tend to think of it. 

Constant change can be kept in check by selection against it or it can be tolerated and incorporated into a lineage, which is usually the case. If change wasn't tolerated by selection, we'd have stasis in lineages, with clones upon clones upon clones. Instead we have myriad sexually reproducing organisms that do not produce clones. Change clearly works!

All this constant change has got to make us think twice when we describe each and every amazing trait in animals as "adaptive" which is to say they're the result of selection. Perhaps they are, but perhaps adaptive traits are just the tolerated survivors that got through selection's filter. 

In that sense, nearly everything that's not a dead-end can be considered "adaptive" at any given moment. That is, all the underlying processes, quantum, molecular, chemical and physiological that keep an organism running could be considered adaptive at any point of observation. 

Inherited genetic material that's involved in growth, development, metabolism, reproduction, etc... (i.e. all life's processes) is evolving with each and every generation thanks to recombining parents' genes, mutations, genetic drift and selection all working always together. Yet, organisms are continuing to survive and reproduce like pros. 

And we have been for 4 billion  years. 

Everything is because of what came before. 

That's all evolution is.  

And that's all we need.