
Our Early Ancestors Stalked Eight-Foot-Tall Sloths - Thevet
https://www.newyorker.com/elements/lab-notes/our-early-ancestors-stalked-terrifying-eight-foot-tall-sloths
======
dexen
Alternatively it could be argued that increasing temperatures and abundance of
food favored smaller animals (including our very low-tech ancestors), which
pushed the proverbial _mammoths_ out of their ecological niche and into the
oblivion.

The smaller the animal, the larger _skin_surface_ : _body_mass_ ratio, thus
higher heat loss. The smaller the animal, the shorter the intervals between
feedings it can tolerate. A large-bodied animal can go for longer period of
time without food, especially if equipped with slow metabolism. In contrast,
smaller body and faster metabolism requires frequent feeding, but also allows
an animal to out-compete slower, larger ones both in food search, and by out-
breeding them.

For any given body plan, an animal can only cover distance roughly relative to
its body size; the smaller animal, the harder time it will have to find
sufficient food on sparse, hardy cold-climate vegetation. On the other hand,
once denser, warm climate vegetation takes hold, it allows smaller herbivores
to live in higher density, and with higher breeding rate.

If I remember my biology right, the more an animal has specializations for
particular environment or niche, the slower it will adapt to changed
circumstances, both in body build, and in behavior & seasonal migration
patterns.

Last but not least, a predator population requires certain density of prey to
be sustainable. Once plant food becomes abundant due to warming up, and
herbivore population grows, the population of predators can be established,
and cull the large bodied animals unadapted to heavy predation.

~~~
stevecalifornia
The reason we believe that humans wiped out the mega-fauna is because the
megafauna of North America all disappeared within a millennia of humans
arriving from the land-bridge. It's possible they all coincidentally
disappeared but unlikely.

~~~
vivekd
I don't consider it unlikely at all considering that the human entrance into
N. America coincides with huge climate change events - the Younger Dryas cold
snap followed by the abrupt and quick Holocene warming event. It's even
possible that the human migration was a result of looking for favorable
conditions as climate change made their traditional hunting and foraging areas
untenable.

~~~
mnort
#stoned_ape_theory

------
MarkG509
But, when did Sloths lose their energy? I have seen videos[1] of Sloths
crawling/slithering slowly across a road, and cannot imagine they were ever a
formidable or challenging target for (pre-)humans.

[1]
[https://www.youtube.com/watch?v=ES32UFlPOUA](https://www.youtube.com/watch?v=ES32UFlPOUA)

~~~
nkrisc
I don't know the answer to your question but I wouldn't compare modern sloths
to ancient ones. They're each adapted for completely different environments.

~~~
oregondan
That's exactly the point of the question, though: when/how did sloths
transition from "ancient" to "modern"? If ancient sloths were massive and
formidable prey, as per the article, what happened? Were there always smaller
and/or slower sloth varieties, and we just killed all the giant ones?

EDIT: As always, Wikipedia comes through [1]. There were species of different
sizes, with tree sloths always being smaller than ground sloths [2], the giant
of the article

[1]
[https://en.wikipedia.org/wiki/Sloth#Evolution](https://en.wikipedia.org/wiki/Sloth#Evolution)
[2]
[https://en.wikipedia.org/wiki/Ground_sloth](https://en.wikipedia.org/wiki/Ground_sloth)

------
anon949714
There is decent evidence recently on a possible astroid impact and massive
"flood" overland tsunami from the Arctic.
[https://en.wikipedia.org/wiki/Younger_Dryas_impact_hypothesi...](https://en.wikipedia.org/wiki/Younger_Dryas_impact_hypothesis)

------
baxtr
We have a “great” history for killing all big animals. So sad. But I guess
they’ll grow back once we’re gone

[https://www.theatlantic.com/science/archive/2018/04/in-a-
few...](https://www.theatlantic.com/science/archive/2018/04/in-a-few-
centuries-cows-could-be-the-largest-land-animals-left/558323/)

~~~
twodave
We have been in competition with the animals on this planet since we arrived.
Commercialization has certainly caused some avoidable consequences to animal
populations, and I agree we ought to take care of the earth as well as we can,
but I just can’t get worked up on the animals’ behalf over our ancient
ancestors killing them and using basically every part of the carcass in an
effort to survive.

~~~
titzer
Oh don't worry, no one is expecting you to get worked up about it. It's just
that we all tend to have this attitude that we are the pinnacle of nature's
great evolutionary process, when pretty much all evidence suggests we're a
bunch of bastards to most life on Earth--and each other, incidentally. Go
humans?

~~~
adrianN
All creatures try their hardest to out-compete everything else. It's just that
we're very good at it.

On the other hand, we're also pretty much the only chance for terrestic life
to survive the end of our solar system.

~~~
titzer
> All creatures try their hardest to out-compete everything else.

When we take a wider look at life, this just isn't universal. If you look at
ecosystems in very stable environments (e.g. caves), the ecosystem as a whole
tends to come into balance over time, where species are not aggressively try
to outcompete their competitors, but instead fit into comfortable little
niches so that the system as a whole keeps functioning well. In fact,
evolution works on the ecosystem scale as well, and systems with aggressive
species tend to be very unstable and prone to crashes. In environments with
constant change, then ecosystems are constantly changing and competition
becomes more fierce, benefiting aggressive species. This is not always the
case, so I don't think it's useful to assume that everything is trying to move
as aggressively as possible; otherwise evolution would select against species
with long gestation periods and tend towards shorter generations and faster
growth.

~~~
jessaustin
There isn't any particular reason to privilege caves or other "stable"
habitats. This is an incomplete understanding of them, anyway. Because caves
are nutrient-poor, it is common that all animals there are extremely
lethargic. That is not the same as lacking aggression or competition. Cave
animals strive to survive as fiercely as anything in the jungle, just at a
much slower pace. If a more active predator were to be stranded in a cave, it
might eat everything it can find in a week, and then starve. Within months,
other animals from more remote corners of the cave would scavenge its carcass
and repopulate.

To visualize this, watch one of those speeded-up videos of starfish and sea
urchins. At our pace, they are sedentary. At their pace, it's Thunderdome all
day every day.

~~~
titzer
Caves were just an example, and pretty much the rest of your comment actually
proves my point. If a "more active predator" enters a cave and subsequently
eats everything and dies, then evolution has selected against it. Evolution
selects species that best fit the environment, and that generally means
species that find a sustainable balance through both internal and external
regulation mechanisms.

Your example doesn't require an invasive species. Aggressive species (ones
with their consumption and reproduction clocks set "too high") can arise
through mutation, too. In either case they spread like cancer and usually take
down parts of the ecosystem with them before they are finally selected
against. Eventually, ecosystems find mechanisms to limit the damage of cancer-
like species and adjust everyone's "clocks" to appropriate levels. Ecosystems
that cannot bring their species into stasis experience repeated shocks, are
less stable than ones that do find stasis, and often disappear. This is
exactly why the long arc of evolution is best understood as a punctuated
equilibrium
([https://en.wikipedia.org/wiki/Punctuated_equilibrium](https://en.wikipedia.org/wiki/Punctuated_equilibrium))
rather than a constant drift. Systems that find stasis are more successful
than ones that don't. Unfortunately these systems are repeatedly interrupted
by drastic environmental changes like geological events. So, the systems that
survive are the ones that tend toward balance but are able to adapt to
occasional disruptive events. That requires biodiversity that is generally
reduced by aggressive species.

> To visualize this, watch one of those speeded-up videos of starfish and sea
> urchins. At our pace, they are sedentary. At their pace, it's Thunderdome
> all day every day.

Or look at hardwood forests that develop extremely slowly over thousands of
years, despite the availability of plenty of energy from the sun. The slower
the system, the better it actually proves my point. Clearly, evolution does
not select for the fastest growing, most aggressive species out there.
Otherwise starfish and sea urchins would move faster, or be displaced by
something that moved faster.

Properly understood, ecosystems are super organisms that require many many
moving parts to bootstrap, grow and function. Human bodies are analogous. Your
cells aren't trying to "outcompete" every other cell in a Thunderdome
situation every day. They are carefully regulated both internally and
externally. When those regulatory mechanisms fail, you end up with cancer and
you die. Clearly, cell replication rate is a key regulator internal to a cell
that prevents us all from experiencing runaway cancer and dying. Clearly
ecosystems have species with internal regulation mechanisms as well.

------
gruglife
Big foot did exist

