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Homo Luzonensis

New species of ancient human discovered in Philippines cave

Homo luzonensis fossils found in Luzon island cave,dating back up to 67,000 years

A new species of ancient human, thought to have been under 4ft tall and adapted to climbing trees, has been discovered in the Philippines, providing a twist in the story of human evolution.

The specimen, named Homo luzonensis, was excavated from Callao cave on Luzon island in the northern Philippines and has been dated to 50,000-67,000 years ago – when modern humans and the Neanderthals were spreading across Europe and into Asia.

[The Guardian]

See also Homo floresiensis

Out of Asia

H.luzonensis lived on the island of Luzon in the Philippines prior to 50 ka. The partial remains of at least 3 individuals (teeth, foot & hand bones, partial femur) were uncovered in Callao Cave. These bones add to a single foot-bone found in the cave in 2007, dated to 67 ka, allowing to make a formal diagnosis of the new species. Alongside H.floresiensis, the discovery of H.luzonensis underscores the significance of island populations in SE.Asia in the evolution of the genus Homo. Although the 2 spp were different, their situations were possibly similar: relics of populations of earlier hominins marooned on remote islands, and following their own evolutionary trajectories. [Nature 568.11.4.19]

[Nature]

A new species of Homo from the Late Pleistocene of the Philippines

Florent Détroit, Armand Salvador Mijares, Julien Corny, Guillaume Daver, Clément Zanolli, Eusebio Dizon, Emil Robles, Rainer Grün & Philip Piper
2019 Nature 568:181­:6

A hominin 3rd metatarsal discovered in 2007 in Callao Cave (N-Luzon), dated to 67 ka, provided the earliest direct evidence of a human presence
in the Philippines. Analysis of this foot-bone suggested that it belonged to the genus Homo, but to which species was unclear.

Here we report the discovery of 12 additional hominin elements, that represent at least 3 individuals, found in the same stratigraphic layer of
Callao Cave as the previously discovered MT. These specimens display a combination of primitive & derived morphological features, different from the combination of features found in other Homo spp (incl. H.floresiensis & H.sapiens), this warrants their attribution to a new species, which we name
H.luzonensis.

The presence of another & previously unknown Late-Pleistocene hominin species East of the Wallace Line underscores the importance of island
SE.Asia in the evolution of the genus Homo. [Nature]

This year anthropologists found a new dwarfed human species in the Philippines: H.luzonensis.

So why did tiny humans wind up living on these islands? For us bio-geographers & evolutionary biologists, the answer was right in front of us: the island rule. Island life & body size Zoologist J Bristol Foster originally proposed the island rule in 1964:

-when a large-bodied species settles onto an island, it will tend to evolve to shrink in size — all the way to the point of leaving dwarf descendants,
-small-bodied spp will evolve to be larger, producing gigantic daughter species.

There are spectacular cases of this island rule in action across the world, e.g.
-pygmy elephants & mammoths from Mediterranean & Baja California islands,
-hippos that would barely outweigh a donkey in Cyprus,
-deer as tall as a pet dog in Crete,
-rats as big as a cow in the Caribbean,
-insects as long as a human hand in New Zealand.

Biologists have proposed various mechanisms that could be responsible for this evolutionary trend, e.g. the absence of natural predators on islands. A number of spp (e.g. elephants & hippos) fend predators off by virtue of their size, an expensive strategy when no killer is lurking in the dark. Also, on islands the scarce resource supply might favor smaller body size, because smaller individuals can live with less. Or it could be that smaller individuals with no predators just produce more offspring, which implies females start delivering earlier & at smaller size, investing less in growth & more in reproduction. This possibility is a likely explanation for how contemporary human pygmies evolved. All of these options will eventually lead to changes in the genetic architecture that underlies body-size variation. So could the island rule be an explanation for small size of H.floresiensis & H.luzonensis?
We thought probably yes.

Modeling generations on the island

The Hobbit's most likely ancestor is H.erectus, a species more than twice its size in terms of its brain & overall bulk. Based on the geological history of Flores & the oldest known fossils of H.floresiensis, it seems the evolution of the new species must have occurred in < 300 ky. As evolutionary biologists, we are acquainted with the idea that Darwinian evolution is a slow & gradual process, that takes place over very long timescales.
Could such drastic change in body size happen this fast?

Our interdisciplinary research team developed a computer model. It's like a computer game that simulates body size evolution under biologically & ecologically realistic scenarios. In our model, individuals colonize the island, grow to their adult body size according to how much food is available, give birth to a number of young, and die. The basic rule of the game is that individuals that are closer to the "optimum" body size for the island in that moment will leave more descendants. Offspring inherit genes for large or small body size. Generation after generation, new mutations may appear in the population, and shift body size toward either higher or lower values. Occasionally, new individuals might even invade the island, and mix with the residents.

Another basic rule is that the initial small population cannot grow above the number the island's resources might sustain. Our colleagues, Earth systems scientists Neil Edwards & Phil Holden, used paleo-climatic data to tweak our model. Hotter & wetter times can support more people on the island, and would influence optimum body size at any given moment. We started our simulations, assuming that large-bodied H.erectus arrived at the island, and then evolved into a smaller species there. Since we just don't know the exact numbers our model should crank through, we based them on estimates obtained from current human populations. Because of this uncertainty, we ran our model 1000s of times, each time using a random combination of all the parameters. Ultimately, we were able to build a statistical distribution of how long it took for H.erectus to become as small as H.floresiensis.

A new species, in the blink of an evolutionary eye

After running 10,000 simulations, we were surprised to discover that in < 350 generations, the process was complete. Thinking in terms of years, assuming a young female delivers a first baby at the average age of 15, that translates to c 10 ky. That may seem long for you and me, but from an evolutionary perspective, that's the blink of an eye — a little more than a 1000th of Homo evolutionary history. Of course, we do not expect that all the features that make H.floresiensis as unique as it is evolved that fast & at the same time. Yet, our simulation still shows, 300 ky is far more than enough time for a new human species to arise.

Our work supports:
-fast evolution is quite plausible under a realistic set of ecological parameters,
-natural selection may be a powerful force influencing body size on islands.
And if H.floresiensis is indeed a product of the island rule, she shows — yet again — that we humans tend to obey the same overall rules driving evolution in many other mammals.

José Alexandre F. Diniz-Filho cs 2019
Quantitative genetics of body size evolution on islands: an individual-based simulation approach
Biol.Lett.15 doi 10.1098/rsbl.2019.0481

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